What privacy concerns (what can a relay see from users connections and posts? IP address etc.)?

How long are relays keeping notes (weeks, months, forever?)

Do Relay's talk/sync with other relays?

Spam

If someone wanted to host their own private relay for backup where are some good resources for the technically adept (well, somewhat adept)?

Any storage and bandwidth stats. Would be super interesting.

With the network growth, how are operating costs holding up? Will we need monthly subscriptions to relays to keep things going in the future?