Overall yes the planes are unbalanced and are broken at times.
fighters are not that great against ground forces and in irl fighters were used to strafe ground units. but the main problem is that planes do not take/deal normal damage. example 1 fighter could kill 5 fighters without getting damaged. one could say this is realistic others would say no.
There was an option for players to vote in the past about adding in Call of War plane mechanics into S1914. but the community shot that down.
as with split stacks is a bit tricky, there is a 4 min window that you are able to change target right after a split stack. there was a suggestion about having code that would identify a split stack so planes couldn't re-target better units in the area.
all units have a hidden range there is splash damage that is about 5pixels same as with the melee range.
overall it might be best just to come with different mechanics than trying to fix the current. its very confusing units in the game. i did suggest have AA guns to help fight against the op planes.