MFA and allowing some sort of music streaming (specifically Plex, for me) would make this a no brainer. Without those two, like you said, it wouldn't be able to be my only mobile device.
Exactly! The Minimal Phone[0] seems to be headed in a better direction in this sense, as the 'limitation' of that device is the screen being eink, but the OS is more or less vanila Android.
Interestingly, I got used to using my regular LineageOS phone in reading mode (greyscale) with a similar effect: I used it less and my usage is more on the point than before.
[0] minimalcompany.com/