I used the HDMI cable provided with my monitor, I've used this cable on many devices and have had no such issues before.
I've tried it on a 5v3A charger, a Sony 5v2.1A and an Adafruit 5.1v4A adaptor. Same result every time.
I do not have any other monitor right now, but a few things that I have noticed.
It is maximum when there is good contrast on screen ie parts of the screen are very bright and parts of the screen are dark.
I thing is that it is due to some issue with non-hdr monitor not responding well to the hdr signals, I've noticed that the newer ubuntu image has buch deeper black so i am guessing hdr stuff is enabled.
Armbian images like the one that boots using the sdcard seem to work fine, since i guess they lack hdr stuff?
Maybe have a way to read the edid at boot and revert back to hdmi 1.4 if monitor without hdmi2.0m support is detected?