Yeah, you can buy the potions of respec and they’re fairly cheap. You can also obtain all witcher sets (and later upgrade them) so you can basically respec and try different builds at will, as long as you sell stuff to merchants at least every once in a while.
As for nerfing your character - the talent tree is painfully simple and it takes ages before you get to any interesting talents, even if you focus on one school. By the time you get to the part where you’d actually see any impact from your talent choices you’ll probably know what you want to play as.