Full text search has been merged in #Mastodon main
branch, and will be in the next (and final?) 4.2.0 beta 🎉 It is opt-in, so it will take some time to be filled with people content as they enable their profile to be indexed, but this was one of the most wanted Mastodon features for some time. We plan to deploy it to mastodon.social and mastodon.online in the coming days to have a bit more feedback on it and see how it behaves in the wild.
hashtags are supposed to be an optimization for search, not the sole axis of searchability.