Welcome welcome!
From what I know in bits and pieces, FE5 hacking is extremely painful to do and there’s limited documentation. GBA has the most documentation and support, albeit you’d be losing the Thracian mechanics unless you dig deeper and set up the patches for it.
FEBuilder is the program that I’d highly recommend if you want to delve into GBA development. Ultra created a FAQ for common FEBuilder problems you may find during development. And of course, I would recommend joining the discord server for additional help and whatnot
Good luck!
