I think for monks control groups are just not useful. Cotrol groups are - as the name suggests - for groups. Right opposite Monk micro is individual unit micro.
Some days ago I suggested a “next unit of the same type” hotkey.
But it’s clear that whenever this is introduced it must come with a slight nerf to monks and possibly also some adjustments to certain civs like aztecs.
And to whoever claims monk micro was easy… I don’t know in what world you live in.