Check strfry, I believe it has a mechanism to sync relays.

https://github.com/hoytech/strfry

If optimized, a relay should be able to do Twitter levels of traffic by itself.

https://thume.ca/2023/01/02/one-machine-twitter/

The fact is that hardware improves exponentially faster than the size of humanity, so we are probably at the point where single-node Twitter is feasible. That's a first, and what may change things.