I suspect your Thunderbolt cable is bad on your Display #1 Its also possible your power supply is going as well so the logic board is not getting good power for the USB ports.
While I know you want two displays working here lets see if we can use parts from one to diagnose the other.
I’m going to suspect if you where to swap the displays the system which has the dirt on the backlight and has the fan running hard will become quiet with the other display (that would imply the thermal sensor has an issue or the screen is running hot).
If you were to take the TB cable from the good system #2 and put it on system #1 the brightness control should show up and stay, if not then its the power supply.