I have an issue because I am creating a modpack with optional mods.

I have Phosphor (https://github.com/jellysquid3/Phosphor) as an optional mod as well (so players can disable it easily to see if some bug might be caused by it) they ship Mixin as well.
I COULD explain this to my players and provide both fat and thin but I'm hoping to avoid that if there is something I can do about it.

Could I just go to their github and build it and drop it in the mods folder and just ship the thin version?

Nevermind I seem to be having this issue with THIN as well, something else must be going on.