The combat system in Civ4 was deeper than you think. Stack composition, terrain, and positioning are crucial in MP games. This write-up [1] from a famous MP game where a 3v1 invasion was repelled by superior play shows how good the system was.
It had depth, but no tactical combat. A stack is an operational unit. Tactics deals with what the individual units within the stack do once the fighting starts. Civ 7 is supposed to introduce commanders, which are effectively stacks for moving troops combined with more specialized great generals. You can get the troops more easily to the battlefield, but individual units still need to occupy separate tiles in the battle.
[1] https://sullla.com/Civ4/RBPB2-5.html