You are talking about a 6 figure number to get "real" international protection. And to defend it could go into the mid 6 figures or higher. And then you could still get knocked off .... Totally not feasible, IMO for an individual. Just get it produced and don't worry about an NDC.
I've known of many designers getting their products mass produced by aligning with a national rep group, that has showrooms in the gift marts.
Example: (this is only 1 example .... there are numerous, only listing this one because I knew the 2 ceramic artists were there)
First 4 vendors listed in this showroom are artists from MS, AL & Georgia. (2 of them are painted ceramics, not pottery) They come up with the designs and Caffco gets them produced in China. And sells them. A rep group typically charges 15% for selling your products. Many of these rep groups have extensive experience overseas manufacturing and importing.
At the present time I have no interest in getting any part of my line reproduced in China (and yes I've been approached by 3 or 4 show rooms). BUT if I ever did, this is the route I would go. I do know quite a few people that have gone this route and have been very satisfied. And some ..... not so satisfied.
Interesting account of that person doing everything themselves and selling from a website. Definitely workable in today's internet and social media mecca ... but this would be a daunting task. And would require more upfront dollar investment. So at least consider using the expertise of a rep group/show room. (and there are quite a few considerations to be made here too)
As for knocking you off, a good rep group can get your product out quickly. It's more about distribution than the designs. And being first.
Everyone ... and I do mean everyone ... that shows in Atlanta (or any major show ... I just use Atlanta because I worked there) has the chance of getting knocked off. Same thing with overseas production. You just can't let anyone outpace you in getting your product out.