There's a free app called "spacedesk" that turns a "secondary machine" into a second monitor. Your main computer has to run on Windows, but the secondary device can be an Android, a Windows or a Mac one. Both machines must be on the same LAN. All info is on https://spacedesk.net/
I have one monitor only and really missed being able to get a fullscreen preview. The tablet I tried as a secondary display has a 10" screen (only) and isn't 16:9. So getting a fullscreen preview on isn't very interesting, plus the colors are different than on my main monitor...
But, the trick is, I can get a fullscreen preview on my main display! I reduce HF's window, put it on the tablet, make it fullscreen again, THEN go to "options" and there "set screen" to my main monitor I could even do this with my phone (if I only had my phone). I tried. I had to use a magnifier, but it worked and I was able to get a full screen preview on my main monitor. Of course neither of these are a practical working set-up, but they do allow for a really full screen preview.
I also tried with a laptop I have. It's cheap and can't do much more than word processing. But it's screen is nice and makes a very decent secondary display I can send ther fullscreen preview to, or it can host the workspace so the full screen preview can be on my usual monitor. It's just that don't have enough room on my desk to make that a permanent solution.