> I'd like to have an analogue to jump back with my C-x stuff like I do with M-. and M-, - any emacs people have suggestions on how to do that?

If you use Xref UI for "Find References/Implementations/Type", M-, should work in those cases too.

There is a more general question: how to "jump forward" again, without re-invoking the previous navigation command with the exact arguments. IDEA, already mentioned in comments, has key bindings for that.

There are several third-party packages which attempt to solve it as well. I'm using this one:

https://github.com/tcw165/history

You can also add "jump back" to your other navigation commands, even if they don't use the Xref UI.