Mirror a specific desktop

I have an external monitor (a projector wired via HDMI). I have been looking for too long of a time not to ask here: is there any solution to only mirror a specific desktop?

I am a teacher, and I would like to have one desktop mirroring what I’m doing, while still having the liberty to navigate to other desktops and these would only be visible to me.

Basically, none of that “press option” in the Display menu in the Preferences would work, because that is effective if you work on multiple displays, not multiple desktops.

Closest thing I found was this Second screen mirror one desktop but no solution was found. If there is a paid app, I’m willing to pay also. Can’t continue breaking my neck (so far I’ve been extending my desktop so I could also see what the students see, without having to mirror desktops).