Hi @paul !
>relays
I don't know how relays work in Mastodon, but in Pleroma they simply boost posts. You can create a separate account on your Mitra server and follow relays from there. Relayed posts will appear in your federated timeline, but not in your personal home feed. This is what I'm currently doing at my own instance.
>bookmarks
Agreed, this feature would be nice to have.