Most instances have an open directory which lets you browse through the most active/recent profiles, but yeah, bootstrapping any new network is difficult. Even more so due to the fragmentation of the fediverse.
In any case, I have running and managing a instance of Mastodon (and Matrix, and XMPP) as a side-project for some time. Some of my users asked about having a twitter bridge, where they could at least follow who they are used to from one single client. Would that help you?
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.
In any case, I have running and managing a instance of Mastodon (and Matrix, and XMPP) as a side-project for some time. Some of my users asked about having a twitter bridge, where they could at least follow who they are used to from one single client. Would that help you?