You can simply create two dummies and mirror each one to separate displays. Or you can use the 4K display as is (since it will natively have 1920x1080 HiDPI on M1) and create a dummy to use with the QHD display to get 1920x1080 HiDPI.
Especially with the wholly Apple-made M series of chips, they control the entire video stack down to the transistor level.
Dummy plugs should be syscalls, not physical things.
I agree with you, it is funny that we need dummies and workarounds (being hardware or software).