How do I backup custom actions (and perhaps other custom changes I've made) so I can restore it on another Windows PC?

General Discussion
1

as above.

if I backed up my config, will that include custom actions? thanks

2

If you save your Mp3tag configuration with File -> Save configuration, the Action Groups are part of the "Mp3tagSettings.zip"-Archive.

Just have a look inside the *.zip file and you will find all of them in the subfolder \data\actions

So the answer is:
YES, custom actions are also included.

3

I don't know how you did it - as you don't tell.

The documenation has a section about the preferred way: