For anyone interested: I've been using a large 144Hz 2560x1440 monitor next to a pair of 60Hz 1920x1080 ones with only a touch of xrandr (one line per screen) to make the positioning comfortable for me (just as a matter of preference, due to monitor height differences).

Idunno when this person last checked (and seemingly then set a permanent opinion, as many do for some reason) but getting X to handle differing refresh rates in multi-monitor setups is trivial now, unless I'm missing something.