Totally fine to have to have a longer legal name but go by a shorter nickname in your daily life
I don't think that's pretentious AT ALL. Thinking about how potential shortenings work is like, part of how folks pick names when they do it the usual way!
Besides, you gotta live with that name for a long time so you wanna make sure it's what YOU want.
Also worth pointing out that if you have a short name (like mine) people will always be asking you what it's short for so you may as well do what you want lol
@rose My legal name is Arthur, but I prefer to go by Art (how cool is "Art Smart"). But it's really a pain to have a legal name that's different from what I prefer to be called
If you prefer to be called Rose, then I would make that the legal name. You'll be glad you did.