Medics are meant to be for healing people. Personally, I believe the mark of a good healing (not tanking) medic is having the least kills by a large margin.
If you get bored in the later waves, play on suicidal or switch to a perk that's meant to kill stuff.
That said, I think a next-tier medic gun should have better healing capabilities, and not better offensive capabilities. Could you detail what you think would be good improvements to the dart? I'd rather see improvements to the dart than improvements to damage or controllability.
Just my two cents. Mind, I pretty much never play as medic, on my current account or my friend's account before I bought the game. So, if what I'm saying doesn't make sense to experienced medics, then I apologize.