At least for mastodon, you should be able to subscribe to a relay server to push posts to your instance. I’m not sure if there are open ones. Last time I checked, I couldn’t find one still working (ping me if you find one).
A relay server will push all the messages it knows about to your server so your federated timeline will be less empty.
The organic way to do it is to follow as many accounts as you can and wait for their boosts to come through your instance. The only problem with it is that you usually get the parent posts a a few replies and you will miss 90% of the conversation.