Alright, follow all the steps I listed in this
LinkIncluding the twiddling the thumbs part before installing the UPnP Media Interface plugin, you'd think I would have remembered that by now....
Except instead of adding perl.exe to the firewall rules. Add the following file
C:\Program Files (x86)\Squeezebox\server\SqueezeSvr.exe
After restarting the service, squeezeplayer and musicbee it's all working fine for me.
edit: Also just a PSA, it's absolutely mental how many firewall rules were listed for apps I uninstalled years ago and some of them don't even display when you open the firewall settings from the Windows Security system tray icon. You have to go to control panel->windows defender firewall->advanced settings. Clearly the app uninstallers are not removing them like they should.
edit: and now I've uninstalled everything I just found an outbound port rule still in the firewall for 49382 that I entered the other day and thought I had deleted... so if you follow all the steps and it's still not working. Allow TCP outbound on port 49382 and see if that helps.