I agree that fit is a big problem here. Usually suits end up loose around my core. Are there brands to look for that might be more likely to work for my shape? Or custom fits in my budget?
That said, even when things fit, I still need warmer than others (i.e. in carribean waters where others dive in just their swim trunks I'm in a shortie or thin full length to be comfortable).
Unfortunately the budget really is tight so cheaper stop would be helpful.
Dano's gear tends to be budget-friendly: Wetsuits - Spearfishing Wetsuits - MAKO Spearguns but chances are you'll want the "L" length and "M" waist. As others said, open-cell neoprene is the warmest you can get -- provided it fits -- but you sound like you might want to put an extra hooded vest on top and maybe a pair of "tech" shorts too. That together with a custom-fit open-cell, will likely get you close to, if not over, the drysuit price.
You're not going to grow really useful bioprene unless you can jump into a time machine and take up swimming at/before the age of 6: swimmers tend to both develop better cold tolerance and grow subcutaneous fat to insulate us, but you have to start early and do it for a decade+.
You could try contrast showers, they do increase cold tolerance (among other things). They won't stop you from going hypothermic, but they could help with feeling cold and miserable. And they're cheap and are supposed to be good for you anyway.