I love my UR28M, but I’ve gotten so used to (and happy with) its hardware mono button that its lack of effect on the headphones is acutely noticed!
Yeah, I love the control room, the Listen button is to die for, and setting levels using its EBU-compliant output monitoring is great also. Your post tells me I need to poke around in it again (it’s been a while) and see what else I may have forgotten about it!
Another solution, use of the native mono-to-stereo plugin, set to mono output… I use it to make bass and kick mono, but it can also be used on the master bus to listen to the whole mix in mono… especially if you do not have CB Pro with the control room…
Off-topic, but I love being able to setup separate mixes for the monitors and the cans and then being able to switch between them via the Power button on each mix in the Control Room GUI. Much faster than diddling with knobs on my audio interface.