I'm working on a modpack an would like to disable cosmetics and have them pre-set to fit the vibe. Unfortunately, the armor has no cosmetics enabled and as such is invisible upon crafting. Is there any way to make the default cosmetics actually be on by default? I'm on 1.20.1, forge

IIRC, if you open the other menu, there are three tabs.
On the third tab, you can toggle on and off the visibility of the various armor pieces as well as set the color for the various bits on each individual piece.
Your settings in that menu get saved to a file in the config folder, so if you set your design for yourself, found that file, and put it in your server pack, that would I believe cause anyone who loaded in first time to have that setup by default.

Keep in mind it's been a while since I've Minecrafted at all so things may have changed since 1.18