Setting up new iPhone but music is on an older iTunes

Good afternoon,

In anticipation of the new iPhone I am wondering how I go about getting my music off of a 2011 iMac that is running High Sierra. Currently when I plug my iPhone in it tells me my phone needs an update and when I select that update I get a server error. I've tried different cables and different networks to download it. In reading older posts it was suggested to install iTunes 12.8.3.1 on top of that version and it would fix the issue but it doesn't. It will sync wirelessly and that's been fine but if I connect a new phone to that iTunes it won't allow me to sync wirelessly without first setting up the phone, right? How can I restore from a backup?

I only subscribe to iTunes Match and not Apple Music. How do I get all of my music from over the years either onto another computer (M1 MacBook Air) or on to the new iPhone when it's released.

I thought I posted a version of this last night but I can't seem to find it.