Yes thought about this when reading the README. Sidecar has been better than 3rd party apps in this regard (maybe the same technique) but having no DPI control sucks. Thanks for the links, will try to see how it works.
EDIT: works well with Sidecar -> Mirroring the Dummy display! Lag/latency wise, feels at least the same with native Sidecar. And thanks for the humongous amount of res options, even though I don't need it right now.
I have another usecase: I used to have an off-brand external display that doesn't register HiDPI res to Macs. Bought DisplayResX for that, but maybe this can help. Once again thank you!