If you want I can research the matter in more detail, someone else came up with federated sites like Mastodon nodes and that's another pretty gray area.
> I am little Joe running internet forum about space battles with maybe 5 active users right now and no more than 100 active members historically.
Ok.
> Should I sign data processing agreement with Google because I am using Gmail to send E-mails?
No. You could try to stretch the law to include that particular example but from my reading of it this is perfectly acceptable.
> Should I hire DPO?
No, but you are the de-facto DPO, so if you receive a DSAR then you probably should answer it, though with your user counts I think the chances of that are very small.
> Am I risking my house being taken from me to cover multimilion fine because user posted their photo or e-mail 5 years ago and I’ve missed it because I don’t delete user-posted content together with their account?
No.
But if a regulator should tell you that you should remove a users data (because you refused to for some reason or other) you probably should. The EU does not 'fine first and ask questions later', they will investigate first, warn and then when ignored they will fine. And for a small entity like yours which is more of a hobby than anything else I highly doubt regulators would even bother but you can't rule it out completely. Better increase you comet insurance as well if that's your main worry :)