You don't need to join the instance to see its directory.
When I started my own instance, I went to a handful of different instances and looked at the directory. Then I started following the ones that seemed interesting. That was enough for me to bootstrap my feed to the point of something useful.
which is useless, unless it's a niche instance. mastodon.social is generalist, so most people there are not posting anything I want to see.
But finding a good niche instance is really hard - in terms of the niche, rules, and actually being fast and working well.