I agree. Look at email servers. It just works out. Email server owners don't look at the content. They just host the servers. Both protocols are federated.
Forums will most likely be driven by the community and volunteers. Just move everyone over to the fediverse. Then it should be easier to find such people.
I don't use Gmail. There are plenty of email providers out there that is completely free without ads and privacy focused. Mailfence, Tutanota, ProtonMail etc. Personally I use my ISP provider that is actually pro privacy - Bahnhof . That due it is a niche and if you don't save logs you don't have the log storage cost.
If feddit.nu (only 50 users) did not exist I would have chosen to self-host it on the free Oracle VPS teir.