I'm only just now learning more about federation, so I'm no expert.

Is your instance setup to use NTP? federation requires fairly accurate time from my understanding, and shared hosting normally drifts quite a bit, which might be where your "Header is expired" issue comes from. It could also have something to do with your instance not processing incoming federation stuffs fast enough I think?

There is a chat space at https://matrix.to/#/#lemmy-space:matrix.org with people that run their own instances, which might help answer these questions