"combine" apple IDs?

From the days before iCloud, I had an ID (iTunes) used for music purchases. We'll call that "original."

When additional services arrived (and I can't remember why this decision was made, probably because I thought I would be getting rid of the old address), I created a new Apple ID in the @me.com space.

Over time, and why this wasn't clear to me all along, I have no idea, purchases and subscriptions for some things (Media and Purchases) were associated with the original ID, with other services (iCloud, Messages, FaceTime, etc.) being associated with the new Apple ID.

For some reason up to now, this has worked. Apple Music and Apple TV purchases/rentals went under the original ID, newer things, iPhone app purchases, iCloud Drive, etc, were under the new one.

I recently got a new MacBook, and my old way of doing business seems to no longer work, even though it is working fine on my iPhone 12, iPad Pro, etc. If I log in with original, certain things sync up, but not all. Photos, apple news, iMessage in Cloud, music playlists seem to partially sync.

I contacted support and was told, in essence, the system would "figure it out" and to log in with my original ID and give it time. This did not work, but logging out of original and in with the new Apple ID did end up with a *mostly* right setup.

My goal would be to have everything associated with my original ID magically move over to the new ID, allowing me to easily install new devices, rebuild old ones, etc., without playing this game of signing in and out.

I was told by the same tech support specialist (answered on first ring, never transferred, kudos to Apple) that it would need to be addressed eventually, but in order to get my new MacBook set up and usable, I could continue as is, most would work.

My question is this: if the back end is seeing me as two totally distinct users, then why to certain things work for both IDs?

Suggestions as to whether the larger of problem can be solved without duplicating a bunch of subscriptions (and expense), and how that might work?