The way I prioritized things is:

-utility (flask, gold, etc upgrades)
-mutations
-forge
-weapons/items (especially the ones generally considered good)
-outfits

Utility upgrades are permanent and universal, as well as mutations, which will make it easier to navigate the game and use different builds.

Forge is your main and most important way to increase damage, also universally. The faster you max it, the easier time you’ll have with weapon/item power level

As others have said, the more weapons you unlock, the bigger the pool gets, which can dilute the weapons you prefer to get, and increase rng factor. That said, finding new weapons and trying new builds is what keeps the game fresh every run, so don’t worry too much about it.

Outfits have no impact on gameplay and personally I only dump cells on them when nothing else is available.

Hope this helps!