yeah, maul needs a bit of work.
the suggestions above are a decent way to get it back in balance.
Start with 'you can't maul anything bigger than you', then add 'can't maul anything that can't be knocked prone'.
The above means getting to horses for units, but that should more than cover it for the early game. After that, a damage reduction per maul strike (% damage lost/strike) can work.
And continuing a maul from a backswing should be stopped. Plus, swarm should not be triggered on a maul (first hit yes, but not the maul bonus hits).