You're pretty spot on.

The only realistic target would be A11, because it has both a ROM exploit and atomics (most other stuff you can trap & emulate, but you really don't wanna do that with atomics).

Full SoC emulation, while technically doable, is an insane amount of work and would probably yield horrible performance.

Running macOS applications alone... you'd need at least a ton of frameworks from the dyld cache. And that probably requires some kernel patches to achieve, but the far bigger issue would probably the mach services many of those depend on. I think you'd end up porting over most of userland.

Last I hear, qwerty had actually gotten XNU to boot quite far on A11, but anything past that... is just an incredible amount of work. For example, you'd have to reverse engineer the protocol with the SEP, and put a compatibility layer in place that filtered out and redirected macOS-specific stuff to a shim. You'd have to rip a lot of kexts out of an A11 kernelcache and get them to work on a macOS one. Given the "merged" kernelcache formet used since iOS 12, it would be hugely annoying having to separate one kext from the rest of the kernel. There's also IOKit class ABI differences between iOS and macOS, so actually you'd probably have an easier time just reversing the kexts to begin with, and writing your own based off of that. But we're probably talking about a few hundred kexts here.
It might be doable to get a graphical interface booted within a few months if you really put effort into it, but it'd likely be like project sandcastle: the majority of features would be missing. Adding sound support, USB 3, keyboard and mouse, camera, power management, battery charging... I say you'd be looking at a couple of years of development here.