Possibility to Assign units to group which is not selected by any "select all ... units"

If you tend to use “select all military units” and similar selection functionalities, you know that often there are situations where you need to send few units fight somewhere else, like send 2 trebuchets on the other side of map to attack a castle, but at the same time keep fighting with the rest of your army on different place and keep “selecting all” units (no time for “assign unit to group” when you are micromanaging), so often times you send those trebuchets (or other type of units) on the other side but few seconds later they are going back to the big map bcos you keep selecting them by select all.

WHAT would help?
SO if you could assign units to a special group, like group 9 or something… (bind any keyboard in settings for that special group), those units in that group would not be selected by any “select all” functionalities. This way you could send some micro-mission units backdooring and stuff.

I know it might sound OP because some guys could be backdooring at 10 places at once while pushing with one big army at the front of your base, but give this feature to both sides and you can also DEFEND at many places at once while defending with big army in the middle, etc…

maybe just add possibility to “select all military units which are not in any group” - that might bring the same result

A side from select all military units, and make groups via keyboards, i have on my mouse key binding for select all units in screen and select all military idle units… I never use select all military units because as you said some units can comeback from side-mission