I've got an old Mackie headphone mixer with a monitor matrix that makes it easy to combine discreet outputs with Cuemix (and honestly I rarely use Cuemix now that I round trip through the DAW) but that Mackie has a rough and noisy sound. That is probably why they discontinued it.
I have two other cheap headphone amps. One is Behringer 4 banger and one is a 6 Channel unit I made myself. The home made one sounds best.
I also have a couple dozen 25' headphone extension cords because as you say; it happens where it happens.
It seems to me that many of the satellite mixer units cost as much as the cheap multi channel headphone amps so I am inclined to just use several cheap multi channel headphone amps.
I don't see any need for placing mix controls out with the musicians as it seems for me that you can anticipate most of their needs or preferences and once you are set up that consistency has it's merit.
Another idea is that if you place the control of the monitor mix in the proximity of the musicians they may or may not have the experience to make useful decisions. From my experience with live mixing I have learned that sometimes the person who wants more me isn't playing with the band because they can't hear the rest of the band and so you have to coax them into listening to the whole. I like being able to know what the musicians are hearing so I can make helpful adjustments. For example; if the bass player is diverging from the drums and requests more bass this may or may not be a useful solution. It may be that the bass player needs more kick and less bass in their monitors. I like to listen and try to help make it work.
That's just an opinion and an anecdote. As you know there are many choices and there is something to cater to every desire.
Good luck shopping!!!
:-)
best regards,
mike