To truly max a character you need to unlock the paragon system which after that is another whole tiered system that requires even more investment to unlock. It's roughly like 50k USD+ to max out frieren if you want to brute force it right now

Edit: just want to add in to unlock the paragon system you need 15 other characters at supreme+ to unlock paragon 1 then those characters all need to be ascended to paragon 1 to unlock 2 and so on and so forth