My understanding is that this is the "elephant in the room" right now on federated social platforms.
I set one up as an experiment, and the "out of box experience" wasn't great - there was no equivalent of a default set of servers to pull content in from to fill the federated feed, so there's no real content discovery to get you started. That might be a design choice to avoid a centralised "default" server, fair enough - let's proceed.
Assuming you (or another user on your server) follows some users from other servers, their content then shows up on the "federated" feed. Only posts or re-shares by those users, not anything else from the federation.
From what I see, the only way to really get "seen" in the federation (since there's no other users on your own server) is to interact with and reply to others' posts (or follow them), and some may choose to follow you, since otherwise they won't know your server exists.
I am not sure that you can "federate" an instance in the way you allude to - there's an option to set up relays, but I've never really seen much written about these. It strikes me that to support a network of federated servers, there is still a need for "many-to-many" relaying, in order to help bootstrap a new server instance and get some content in from the wider world.
Maybe all of this is "by design", but I do worry with some of these platforms that we risk creating more isolated echo chambers - it feels like there's a gap at the moment in the setup which makes it more sensible to be a member of a big "centralised" popular instance (like mastodon.social), which then takes everything back to where it all began with a single centralised platform!