i've moved from Fauna to Supabase... their FQL language was kind of confusing and poorly documented compared to Supabase

If Fauna went open-source, I'd make the case to pay for their hosting. There's business value in just having the option to host it internally or make modifications.

I agree. I love Fauna, and I'd definitely use their hosting even if they offered a self hosting option, but the closed source and vendor lock-in are really strong points.