> How do you decide which peers to sync with on Amethyst P2P? There are so many users.

It can be done, here's just  an example: 
It should be possible to generate an onion address keypair algorithmically from the recipient's Nostr keypair, in a manner that is reproducible on sender-side using only the public key. Roughly similar to how Bitcoin HD wallets (xprv/xpub) work.

There are probably some privacy footnotes with this method, but would give you 100% accurate peer discovery without an intermediary. 

Just to be clear, none of this exists. It's only an example of how it could be done.