Check my new post. You can start there. And this one too https://retrotoon.world/signup.php

I see an application requiring screenshots indicating proof of membership in other private trackers

Yes, that means they are currently open. Note that, as this post's title says, they are Open Application (not to be confused with Open Registration), meaning anyone can apply without an invitation.

If you do not have proofs of membership on other PTs, they might still consider you if you give them reasons to let you in. For example, a screenshot of something that shows you would be a valuable member of the community, such as your torrent client showing long seed times and high ratios. Be sure to make your case in the text box.

Good luck.