Actually, the best solution for music purchases is to buy the CD. It costs about the same, you own physical product, It's often higher quality and you can upload it to iTunes. I only buy from iTunes when something is either not available or I just want single songs. I also burn them all to CD so I have physical archives in case Apple tries to ding me for too many copies (mix CDs I like to make).
The Apple ID problem has been going on for years, ever since they made us convert to an email address, but still kept our other account.
So many years later, Apple still has not chosen to address the issue, so we'll have to live with juggling multiple accounts. I tried to link my two using the "Family" mode, but they have this set to first, assume you are adding a child, and second, not allow you to link your other non-email Apple ID. I think Apple is waiting for all the people with old IDs to die - problem solved!
In a perfect world, a solution would be straightforward, but Apple doesn't have a good track record with much of its software. (i.e. Apple Mail).
Still prefer all this headache to a PC, but Apple, can't you try and up your game just a little bit, please? :-)