How to set up OS Sonoma Music App to access music on external drive ?

One option is if you copied or moved your iTunes Media folder to an external drive, you can plug this drive into your new computer.

Then, hold down the <option> key, and then, select the Music app. You will see a pop up window asking you where this Media folder is located. Point to it location on the external drive. This option works the same as it did with iTunes.

You're very welcome! Oops, yes, you did say that you were just planning on upgrading macOS. My mistake.

FYI ... should you still want to use iTunes, you may consider using a Virtual Machine (VM) manager, like Parallels. You can then create a VM of Mojave, and then, run it within the manager when you want to use iTunes. I don't work for Parallels, but have been using their products for years. I use it with multiple VMs of various operating systems to help folks with their questions with those particular systems.