One solution could be to remove stab from all lances over length 150 (This suggestion will definitely spread rage). Couching should be buffed at the same time and lances should be made longer so couching would be viable in combat, not just in backstabbing. In that way there would be spears and extremely short lances that could thrust and long lances that could only couch.
We already have that. The great lance is the longest lance and it's couch only. Personally I would like that one to be a bit longer. (230, 15 less than the long spear and no bonus from stab) .
What we want to have it is for polearms to be in 3 different categories on horse:
1st: Short(ish) polearms (150>) that can stab in full angle but can't couch.
2nd: Medium length polearms (150-190, maybe 200) that can both stab and couch but in a reduced angle.
3rd: Long lances (200<) that are couch only.
This seems reasonable, doesn't it?