nexusmods.com. I guess I really should check the workshop more often if it's therenexus? as someone that uses mods quite a bit, I'll be honest I rarely even check steam workshop I just go right to
Before transition to support two distinct version of the game using mono and IL2CPP at the same time, you may want to check if you couldn't solve modding problem in IL2CPP using Unity Addressables. It can be used to load pre-defined things during runtime, and with a bit of tinkering these pre-defined things may be also compiled at runtime using content from StreamAssets directory and Addressables API.
I personally think that the best way to support modding is to make the game as a actual mod, like Neverwinter I was made - so starting from day 0 with that in mind.
Anyway, best of luck!
The creator of necesse managed to get a workshop page for the mods for his game, and it also uses BepInEx as far as I know, maybe he can help you to do the same with rogue genesia, he's very active and always helping ppl with everything on his discord!