[lxgr]

That's exactly the problem, in a nutshell. Everything is tangled in a big ball of yarn with Apple:

Theoretically the iTunes/App Store/TV account is independent of iCloud – except that it's tangled to Apple Podcasts.

- iMessage used to be mostly standalone (iCloud sync was explicitly optional!) – but not it's tied to iCloud via contact key verification.

- Books is a weird mix of iCloud (for media) and iTunes (for purchases).

- Having my device as a trusted login factor is a complete mess: I still haven't figured out what makes or doesn't make a device "capable of generating authentication codes".

- iTunes subscriptions can somehow only be managed on an Apple device or iTunes – and logging in for that purpose messes up podcasts (see the first point).

At least on macOS, it's possible to make a second account and log in to most of these cleanly, but it's still a hassle compared to e.g. Google's seamless support for multiple accounts in almost all of their products.