This is why I wish there was a ROM that's purely focused on bringing the latest stock Android to as many devices as possible.

Right now it looks like the most popular cross device ROMs are projects like LineageOS and OmniROM, which are excellent ROMs with plenty of compelling features over stock Android, but the modifications they make to implement those extra features do incur a non-trivial cost in terms of resources needed to support newer versions of Android.

I'd be completely willing to give up all of the extra features they offer if it means I can get a cross device ROM that tracks major Android updates much more closely (ideally weeks in delay instead of months).

Maybe something like this already exists?