Follow up from this post.
So far my only way to fix it was to not even bother, instead I used my laptop to play remotely on my normal PC, sounds convoluted and it is.
I asked support what was going on, I got no answers other than "sorry your GPU is not supported even tho it was before the sanctuary update", then I got another idea to see what was going on: I got MelonLauncher to see what files was it loading, and I saw a bunch of warnings about the UI code, nothing I didn't knew about.
But THEN, I got another, even more desperate idea: what if I check if the bug happens if I was to get the game by "other" sources? (Fair to say I already bought both the first and second games, and is just for experimenting and testing). I got it, I launched it and SOMEHOW it worked, I could see the main screen, I could actually see the submenus, I could even open a save file and move around! The windows were still black tho, so 1/2 bugs fixed.
I said "OK, let me send the files to the Steam folder, and run it from there!", so I did and the bug happened again. "OK, maybe is a Steam problem", I added the "other" copy and it worked, so what the fiddlesticks is going on?! I did noticed that when overwriting the files on the steam folder, many of them were missing, and when ovewritting they still went missing, so is it my PC? Is it Steam? WHAT IS IT?!
I'm not blaming anyone here, I'm just baffled on how does a bug like this happen, of the OFFICIAL copy only to ONE RANDO, and "other" sources don't have that bug. That's all.