But key mismatch error when trying to actually connect between two internal systems.

Can you confirm that there are no spaces before or after the key on the client end? Sometimes, copy and pasting can add extra spaces that cause invalid passwords, etc.

I'm not having issues running my self-hosted Rustdesk (docker) externally, but I can't offer much more than that :(

Ok. Found some DNS settings on my router, and fixed the internal domain name "problem" but it's still only internal. If I set my public IP(internally) it doesn't connect.

I can connect to an internal computer from external, even though the client says "not ready".

I cannot connect from internal to external computer. Instantly shows "remote desktop is offline"

This leads me to think that somehow I have something wrong in router settings, or I have a security feature blocking something. I just don't know enough about routing to know where to look.

It seems I may have "fixed"(?) One problem, as internal network connections succeed now (same key, same settings, just restarted the containers a few times and let it sit?)

External connections still show the same. :/