I find it very tedious to form/break/reform groups during a game. My solution was to break free of my MTW habit and stop trying to move my entire army at once while keeping it in formation. During setup I create roughly six groups, by purpose, and I avoid mixing unit types within the group. So one group is my spear wall, I have left and right infantry groups, left and right cav groups, an archer group, etc. Each group I usually give the single line formation order (shift +1) so that I can draw the group on the map in any thickness or orientation that I want.

When I want to move my whole army from point A to point B, I just Ctrl + A to select all and then right click on point B. A press of the space bar will show that if you did nothing else, your army formation will be all jumbled once they got to point B. So while they are marching to the new location, I redraw the groups on the map in the exact place and orientation that I want them to be when they get there.

There is a change from MTW to RTW for the better (IMO) in that unless you are setting waypoints, when you issue a movement order it automatically cancels the previous order. In MTW you needed to either wait for the previous order to complete, or you needed to manually cancel with a halt command (bringing your units to a stop), before you could change the order. So in MTW, when moving my whole army in formation, I frequently had to move/rotate several times until I got it in exactly the position and facing that I wanted it. In Rome, I just draw it on the map where I want it to be....if I don't like it or want to change it, I just redraw it.