@grammargirl If I remember right, I believe @anildash actually modded his for this purpose.
I otherwise see a ton of good suggestions in this thread that beat me to the punch. Luna display will probably be easiest, though you could always do something like Duet Display if you don’t want to buy hardware.
My suggestion would actually be to look at getting an HDMI capture card. People are doing this to plug in Switches on iPads and I imagine it would work with your old Mac, too.
@grammargirl This actually describes the HDMI capture card approach to a T.
The one challenge will be to find one that can use the full resolution of the monitor.
@grammargirl That said, 2019 is still pretty recent, I hope that you set up like a Plex server on it when you’re not using it as a display. :)
@ernie @grammargirl I never actually did it (lack of time) but I did fully research the project
@anildash @grammargirl it feels like a project I should do sometime honestly, though I have a couple of 4K screens I'm fine with