Canada

Granted, Guyana is probably cheap living, but it’s really difficult to live there without being fluent in French (many of the locals speak French as a second language; relying on English to communicate will be incredibly difficult when French is the unifying language there, not English). The entire département has less than 300K people living there.

The main reason I mentioned it would be if I managed to get a job at the space center, or at least one that's aerospace-adjacent.

Remember, the tropical overseas départements are generally financially poor and lack opportunities even for locals. There’s a reason why there aren’t giant populations in these tropical places.

I'm still seriously considering buying a boat and becoming nomadic. It would be more of a "work odd jobs to fund the next few months of cruising" kind of thing, not trying to permanently settle on a particular island. It's just that the timing isn't ideal because I don't yet have the budget for as nice of one as I was hoping for. (Also, it'd be years before I got to French Polynesia anyway, since I'd most likely get the boat in the Southeast US before traveling around the Caribbean and through the Panama Canal.)

Otherwise, yeah, for a more traditional lifestyle + getting EU citizenship, I agree metropolitan France (and ideally Paris specifically) would be my best bet.