Connecting a 2019 iMac to a 2020 M1 MacBook as second screen

You can't, it won't, it don't work.

1. Only some older iMac models supported Target Display Mode.
2. No 2020 Mac, M1 or otherwise can use a supported iMac as a display.

see > https://support.apple.com/en-us/HT204592

Love my M1 Mac mini (macOS 11.2.2 ), but with that much power, etc. frustrated it couldn't leverage the resolution of my 27 inch iMac. 2012. (running macOS 10.15.7)

Here"s my workaround.

1. Screen share the Mac mini from the iMac, (even with the tiny screen due to standard resolution.)
2. In screen sharing session, select Air-Play to my 4K Apple TV. (how could you not have a 4K Apple TV with 12 free months Apple-TV subscription?)
3. In System Preferences, within the shared session for the Mac mini, choose the display to match the Apple TV you selected to Air Play to and you have your choice of up to 3840 X 2160, I believe. Sweet! More than enough Screen real estate to work with until my business earns enough for my 4K monitor!

Hope this helps.

Interested in any potential downsides to this hack.

+1 in short quit wasting your time. Please buy an external display for the MB, your 2019 iMac is a computer and was never designed to be an external display for another device.