I think mounted combat could be implemented if it was limited to one character one the field, like a commander or general type battle brother. That one character would lend more passives to group morale while alive, have increased movement range, and increased damage output but at a cost of only being able to attack forward or some other limitation, and be very disheartening and expensive to the group if slain. Also the horse could be considered a “brother” even though it’s acting in tandem with its rider, meaning all those bonuses would be at a cost to having another brother on the field for support. That would actually balance having multiple horses in battle as well as you would only be able to have six total entities on the field with enhanced passive buffs.
Out of battle they could just provide faster movement at the expense of more rations being used? Or perhaps you would draw more attention to yourself from raiding parties and ambush bandits given the noise and dust etc. of horse travel. Additionally they could be reasonably expensive and a rather large investment, even taking a tole on the group finances when stabled (still have to eat).
As for the group cohesion, maybe it’s too large scale for the skirmish battle type, but I didn’t intend it as being so. Maybe, for example, if you had the phalanx I mentioned earlier you would have passive range defense applied or a boost to shield wall. It could also allow for archer protection but at the cost of archers being unable to fire (line of sight) unless the group broke the circle for a turn to allow them to shoot.
For example this: https://www.youtube.com/watch?v=bpLtXIlkyYA (start the video at 1.25) not medieval, but the tactic would be similar.