New Mac--Authorize/deauthorize questions

I migrated my system from an old iMac to a new iMac running Catalina using TimeMachine, and now I'm trying to tidy up. Two questions:

1) When syncing my iPhone to the new Mac, I got a notice that some songs (many, in fact) are "not authorized on this computer." I seem to be able to click on them in the library and authorize them one by one, but isn't there a global command that will do that?

2) When I'm satisfied that everything is in place, I intend to turn on the old Mac (it's been turned off since the migration) de-authorize it, and wipe the disk. What's the best way to do that? I seem to have the option to remove it from the account in the Apple ID pane of System Preferences on the new Mac, but is that a good option?