"Is there a way to get posts for a hashtag, even for people that aren't followed by someone on your instance?"
Yes, there is! It's called FediBuzz Relay, instructions are here: https://relay.fedi.buzz
You can also find more ways to increase your server's view of the Fedi at https://fedi.tips/using-relays-to-quickly-expand-a-servers-view-of-the-fediverse
By the way, FediBuzz is more specific than general relays. General relays can cause problems for small instances as they take up lots of resources.
