A few small-looking battle plan improvements, that would remove 90% of annoyance from using them, since all that micro can be automated for 90% of the cases:
Create a separation between frontlines and battle plan orders allowing only some of the units assigned to the frontline to participate in the order. Lets say I want 3 artillery divisions to break through a particular province on the front and then say 6 cav divisions to exploit that breakthrough. Let me just create one small offensive line and one spearhead and assign divisions to those orders.
Then maybe have a per frontline setting that's called "order support", which decides how much the parent frontline would support the orders executed by the parts of it. For example my 6 cav divisions are planning to go over say 7 provinces. I have say 4 extra infantry divisions that I want to start flowing in to support the spearhead order just as the breakthrough is accomplished. Also my 3 40 width arty divisions can attack from 2 provinces, but there's another province I can attack from, so I want one of the available parent frontline divisions to attack from there. Then if my breakthrough or spearhead is stalling I may want the support divisions to join in.
Also ideally I want to chain orders given to independent forces in advance, like don't start the spearhead when breakthrough is not accomplished, since you can already do that with orders assigned to the same frontline.
While this looks simple, this is probably not something you can implement in a mod and since this was not implemented in any DLC, I would assume that would require a complete redesign of the battle plan orders system.