Maybe you're already aware, but bluesky doesn't operate with instances like in ActivityPub land.

I've seen many people I believe are using their own PDS, but yes, discoverability is likely better because a relay is meant to aggregate and share all data it can (look up "bluesky firehose").

P.S. Mastodon's devs are part of a new initiative to improve this area of the Fediverse. Because it's so recent, we'll have to wait to see how it goes.