Hi @Vaskerville — sorry for the delayed response. It can be very frustrating trying to find which process is using a given port. I've written a support doc on this topic:
Port 53 is used for DNS resolution, if that helps prompt any thoughts as to what might cause this on your Mac.
In my experience a surprising culprit which sometimes uses port 53 is Apple’s Virtualization Framework, which is used by Docker (in certain configurations/versions) as well as UTM, Tart and the like (also only in some circumstances, as far as my testing has seen).
Let us know how you get on!