RE: it seems like the only way to have a reasonable chance of getting decent results for hashtag searches is to be on the biggest server
Well, yes, and no. You can relay with other Mastodon instances and they will share there traffic with you so that people on your instance will have the same content as on their instances PLUS the traffic from all the other instances that are relaying with them as well. So, your server will see a ton of stuff. No shortage of stuff coming in
There's a number of good relays. I recommend these two at a minimum:
In the Administration section, there's a Relay page where you can put these in.
For the first one, @Jerry is the person to contact if it doesn't enable for you. Not sure how many instances relay with him, but it must be a large number.
For the second one, which currently has 328 instances connected together, you can get more information at https://bigrelay.social/
Kara