I think you're underestimating the (Vet I+) M1 mortar here a bit. Delayed fuse HE shells which are very good at knocking out team weapons and damaging structures. Alternatively, the airburst can wipe models/cripple squads too. These options, albeit at the cost of 20(?) muni, give it temporarily more firepower than heavier guns like the ISG and Pack Howie. Of course the range is a lot less, but it's still a mortar.
THe mortar is very good indeed, but late game it kinda falls off. At least in team games
The range like you mentioned becomes a problem and the heavier artillery on the field at that point makes it get cleared very easily
Idk what the solution is, not that one is necessarily needed. Dont think u can just add a heavier stock option cuz that would be a significant buff. But it does feel like an issue in team games