I think the 300 sats are a good deterrent. But TheForest does accept social notes, right? (At least for "paying" members?)
Ah, on that front... The script kiddies have evolved. AUTH no longer deters them, unfortunately... But it does deter... like 90% of clients on Nostr from writing to and fetching kind 24133, unfortunately.
I mean, AUTH does help a bit, PoW above 28 also helps a bit, but other than charging some sats or a strict WoT I haven't found a good solution for public relays yet.