what is the purpose of trans people in a fantasy world when they can just sex change via magic themselves (without any of the side effects of actual transitioning) or ask an able wizard to do so?

Utterly fucking nothing beyond social media virtue signalling because when in situ it doesn't make any fucking sense due to arcane, divine, and cursed magics literally being able to perform the deed depending on what someone would want to pay for it.

Arcane: pay a wizard to zap you in the crotch, wait no that's electroshock therapy. Unless... ?

Divine: pay a church to pray away your problem.

...

Hold up! ?

Curses: belts like the one in BG2 found veeery early on in the tent after you escape the tutorial dungeon, spell defenses like on the tome Edwin[a] botches while trying to get more UNLIMITED POWAH! ⚡, pissing off Elminster after getting the tome curse was fixed and getting turned back onto Edwina again because you're a fucking moron Edwin, I don't care how high your INT stat is!

Dragons can probably do something too since many can shapeshift and fuck almost as many random mortal races as the in universe Gods do. Also shape-shifting are the druids so Harpers Larpers and otherkin wannabes would probably take a shot before in all likelihood being actually shot when they transform into a wild animal in a residential area.

Or just burned for being a witch or something else since that too happens in BG2 when you first meet Viconia and depending on whether you bother saving the stupid bint.

Being "trans" wouldn't even be a thing.

Indeed it wouldn't because anyone who wanted to be MtF or FtM could get it done and wouldn't be MtF or FtM. They'd just be whatever they didn't start as.

Or dead. D&D at times has a habit of rewarding stupidity with a proverb for the next idiot to try the same thing like "Don't cast cloudkill in a sealed room."