I'm a level 8 mage but craft wands of lower level (ex: lvl 2 for magic missile, lvl 5 for fireball). Same for potions. Can that be changed?

You always craft and brew at the lowest possible caster level. Anything else would be insanely powerful (but, to a certain extent, it could be changed).

It should stay as it is.

I have a PC who can craft wands as a lvl 9 cleric and if every wand was at 9 or even just a bit less they would be too powerful.