itunes on late 2012 hangs for hours / crashes system / error 42110

Maybe I underestimated the problem in the above description because I have been avoiding iTunes for several months. I recently bought a new iPhone 7+ and want to put music on it, and authorize it for testing with xcode, and realize that iTunes is unusable.

I think it started when I upgraded to High Sierra earlier in 2018. THat's when I stopped using iTunes.

Currently, I am using version 10.13.4, and iTunes always opens with error 42110, and then the spinning beach ball for between 30-120 minutes (most often >30 minutes). If I plug in a phone, the error and beach ball re-appear and the whole system becomes unresponsive for hours, or a re-boot, which also can take an hour. CleanMyMac does sometimes give me an option to force quit, which can be a life saver if I need my system to work quickly.

I can also NOT download purchases made on other devices. Even when I see the items that I have purchased (Account --> Family Purchases) I alway get an error message: Could not purchase "Album name". An unknown error occurred (-42032). There was an error in the iTunes Store. Please try again later.

I can also NOT fully syncronize my phone with my iTunes library selections. I don't think an error is shown, it just doesn't work properly - runs through the steps, but only a small number of albums (20% max) are copied across. I have Sync Music selected; and Selected playlists, artists, albums, and genres also selected.

Other common errors: iTunes could not check for an update to the carrier setting for your iPhone. An unknown error occurred (-50).

Is there an alternative way to purchase and manage music across my devices other than iTunes, or are these problems solvable?

Correction, there is an error when syncronizing:

Some of the items in the iTunes library, including "Song name", were not copied to the iPhone "MyPhone name" because you are not authorized for them on this computer.

To authorize this computer for items purchased from the iTunes Store, choose Account > Authorizations > Authorize this computer.

For a list of items that could not be copied, click the triangle below.

100 total problems.

iTunes hangs with this open for over 30 minutes, freezing everything intermittently.

I never de-authorized this computer, and have meanwhile followed the above instructions. I noticed that 5 computers were authorised, so I deauthorised all of them and authorized this computer. It went from 0 computers to one computer authorized. I restarted everything. Nothing changed.

Some of my music purchases, which I made on my daughter's phone, are not visible. Even if visible, cannot download.