This is gonna be long and I might not finish it but here's what I got. This shit is so long that if I get it down to a science I might make a program to automate it.
We will study your character stats, use FFXIV TexTools to export the model parts, Blender to fix it up....
Do you have your character appearance saved? It's not essential but it's very useful to have. It's in the form of a DAT. I hope you backed it up if you created them on an old PC. I'm not sure if you can resave it from an existing character. What I did was use the Shadowbringers Benchmark. I seem to remember there was a way to import your character from the base game but I can't remember. Anyway first you'll need to copy the character DAT to the benchmark folder.