Lilya

@Bill Kielb Thank you again for the explanation. I'll do some tests soon then and see what happens!

@neilestrick Ok, I'll look into that (no pun intended)!

In general, more alumina could stiffen things up. You won’t know till you try and of course increasing the temperature or time at temperature gives you some control over this as well so it even fits with the suggestion to visually confirm the glaze has fully melted. Having a 15:1 ratio - well outside what could be considered normal though can create its own issues RE: glaze melt.  With the Fritt you have, you are sort of stuck with a high starting silica level. I think likely worth the test to see.

Yup, I know exactly what you're talking about. If it were my kiln, I would make a peep hole in it.

Yes definite typo - sorry, from 90/10 probably through 80/20 …… so should have said 85/15. The reasoning was the Si:Al was very high mainly due to the lack of alumina so I was looking for more alumina which ought to be in your Kaolin. As far as melting at or around cone 05/04, boron in the range of 0.45 should get you there. You currently have 0.61 in one recipe and 0.70 in the other so either ought to melt well ……… even with more kaolin as it is likely this will still be greater than 0.45 boron. Additionally The Kaolin will help suspend things and ought to be more economical than the Fritt. So, 10-20% Kaolin ought to still melt, provides alumina which we appear to be short of and could help with suspension.  All probably worth a try as if the end result suits you it is likely a more economical recipe. If you don’t like the result, then no, it’s just not better. Won’t know until you try it though. I think I would test 10%, 15% and 20% Kaolin - all adding up to 100% to see if I liked one over the other.

Does it have peepholes?

Is it a top loading kiln? If so, you should be able to crack the lid just enough to see what's happening without losing much heat. Again, you'd only have to do that for one firing, to figure out what temp that glaze needs to go to.

+1 for what Neil is saying about checking the glaze melt. It's a bit like driving a car with a blacked out windscreen otherwise, you won't know if you've reached your destination otherwise. 
 

@Min
Just an update (I hope you're having a lovely Sunday): I tried 1254+kaolin and FR8+kaolin this afternoon and the results are pretty good! Consistent, glossy finish on both at 1000°C ... slightly more teeny teeny tiny micropits with the FR8, but both are definitely a huge improvement over what I've seen so far with other mixes. The 1254 seems better in general and was easier to brush on, but the difference is marginal.

Neither flowed into carvings as well as the GB and I did get some black holes, but they were only over the smaller carved areas. For the bigger ones, like my roman numerals to mark the tiles, everything looks as it should. I'm going to try again (when my next batch of test tiles has been fired!) and pay special attention to filling in carved areas first. This time I just wanted to see what happened if I brushed as I would've before. I'll aslo try the 1254/FR8 blend and the 75:15 ratio that @Bill Kielb suggested.
So massive massive thanks again to you and to Bill. I'm feeling quite relieved that I'm getting somewhere! :-)

"... silica and alumina kind of act the opposite to boron?"
This is a good question!
It's been almost five years since I first started reading about glazes ...I'll circle back with recommendations for reading in a day or three.
Silica is a main glass former, and the main glaze ingredient, but it doesn't melt at typical pottery kiln temperatures, hence "fluxes" are required to lower the melting point.
Boron is both a flux and a glass former. Most of us working in mid fire (cone 5/6) depend on Boron; those working at lower temperatures depend on Boron more!
Alumina is important for glasses/glazes (glaze is, essentially, glass) for toughness/strength and, as you point out, to stiffen the melt.
Here's an article on fluxes
Flux (digitalfire.com)
Edited to add ("a day or three" slipped to two weeks):
I like Susan Petersen's book The Craft and Art of Clay, particularly the glaze section, which includes an entry on each important ingredient (including colorants) and clear discussion of unity.

@Lilya I believe both recipes will melt at your low temperature. Fr8 would be a visual favorite and boron greater than 0.45  (0.60) ought to melt well. Just a thought, there is room in both recipes for alumina in my view as 15:1 Si:Al is high. You may want to simply test  with a greater percentage EPK to achieve this. So 85% /15% may be worth a try to see if that works well and …… hey it’s probably more economical as well.
Just to add, you may want to stop at Sue McLeod’s site. She has a nice way of explaining glazes, many of her instruction guides are free to download and she does conduct workshops. https://suemcleodceramics.com/

I would not expect the kaolin to be the problem. It could be that the glaze was simply under-fired. IMO, If you're firing quickly like a typical raku firing, the best way to do it is to actually watch the melt rather than relying on a set temp, at least at first. The glaze will bubble up and then settle down and gloss over. At that point it's ready to pull. Note the temp on the pyrometer at that point and then you can fire by temp from then on. I've also found that raku glazes mature at a fairly wide range of temps since many of them are not real glazes, and that the white crackle glazes tend to like more heat than the copper patina types.

@Hulk Thank you very much for the explanation and the link. I realise I have a very long way to go, but I'm grateful that I'm starting to understand things a bit more. So I'll be depending on Boron quite a lot then, given my low temps!

@Min Oh my goodness thank you so much. I am beyond grateful, thank you thank you. And yes, the colours of the program are hideous but seeing everything there side by side helps an enormous amount, whatever colours they are. Thank you.

So having looked some things up, would I be correct in thinking (roughly) that silica and alumina kind of act the opposite to boron? I know I'm hugely oversimplifying here, but yes: boron melts, they stiffen. So looking at those, the 1254 mix is higher in Si and Al than the FR8 mix, but it's also higher in B which might make up for that? Anyway I will try all of these and see what happens, thank you so so much. My last resort was going to be ordering another batch of GB from a different supplier, praying, and then ordering loads if it works (enough to keep me going while I try more alternatives), but fingers crossed something will work here.
Honestly, thank you _/\_

@Min Oh my goodness thank you so much. I am beyond grateful, thank you thank you. And yes, the colours of the program are hideous but seeing everything there side by side helps an enormous amount, whatever colours they are. Thank you.

So having looked some things up, would I be correct in thinking (roughly) that silica and alumina kind of act the opposite to boron? I know I'm hugely oversimplifying here, but yes: boron melts, they stiffen. So looking at those, the 1254 mix is higher in Si and Al than the FR8 mix, but it's also higher in B which might make up for that? Anyway I will try all of these and see what happens, thank you so so much. My last resort was going to be ordering another batch of GB from a different supplier, praying, and then ordering loads if it works (enough to keep me going while I try more alternatives), but fingers crossed something will work here.
Honestly, thank you _/\_

I compared your original Gerstley Borate recipe with nepheline syenite to ones with Solargil FR8 frit and the Ceradel 1254 frit then tried to come up with something as close as possible using what you have available. Roadblock I ran into was getting the silica and alumina as low in the fritted formulas as the original Gerstley Borate recipe.
I found a blend of the two frits, FR8 and C 1254, got the boron (melter) okay but the silica and alumina are a fair bit higher. I could take the kaolin out of the recipe to lower the silica and alumina but then it will be back to sinking like a stone in the bucket so I kept it in. This means it might not melt quite as well but if you are testing glazes it might be worth a try. Other thing to try would be to fire a bit hotter if it appears not quite melted enough.
I also included what the chem looks like if you use just FR8 or C1254 at 90% with kaolin at 10%. I find the format and colours of the glaze calc program I use very jarring, these are not easy to look at. Also, what looks okay on paper doesn't always work in reality so if you try the blended version please just test a small amount first.

 

@Lilya how were they fired?

I am curious if you have used any glaze calculator, even Glazy will do. Regardless of the boron source, you likely will be shooting for (UMF) 0.45 boron or greater to melt at low temperatures. This may allow you more freedom to use local boron sources to at least get the melt established.

May I ask what was your favorite GB recipe and what Frits are available locally?

Hi @Lilya, welcome to the forum!
To make a glaze brushable:
Mix 2 tbsp CMC and 1/4 tsp copper carbonate with 1 gallon of water. Let it sit overnight, then mix well with a submersion blender. When mixing your glaze, substitute the gum syrup for about 1/3 of the water.
I would probably toss the current glaze. Sounds like you've messed with it enough that you can't save it.
The classic 80/20 raku white crackle glaze is just a clear glaze. It was popular because Gerstley was cheap and it suspended very well. The reason it crackles is because of the thermal shock. You can get the same effect from a low fire clear glaze like a simple 90 Frit 3124 and 10 EPK. Just like the original 80/20, you need to apply it somewhat thick to get good crackle. Having some clay in the recipe will make it suspend and brush better than a frit /neph sye blend.

HERE is a possible solution. Or HERE.  Definitely try the Frit 1254 you mentioned, but it's closer to Ferro 3134 than 3124, so it may not work as well.