That's an interesting approach, but how exactly do you coordinate updates with your main system? That is, how do you migrate and merge updates from the "mule" drive into your main HFS+ system without causing problems, such as wiping out user preferences or breaking compatibility with something due to update changes?I've set it up with Mojave running under HFS+ on the internal drive.
T2 functions are disabled (insofar as they can be, using the Startup Security utility).
It boots and runs just fine with HFS+.
If I need to update the OS, I keep a "mule drive" formatted to APFS with a copy of Mojave on it, and Software Update can be used on that drive. But other than for updates, it is never booted or used.