So here's what I consider the biggest problems with monks:
At my table monks have a d10 hit die, and at levels 5/11/17 they get a +1/2/3 to their unarmed attack rolls - but not damage rolls. It's the most straightforward way to the first two issues.
The third issue may require an explanation. I believe stunning strike ruins monks not because it's bad, but because it's too good. The single greatest contribution a monk can make in any encounter is to walk up to the biggest and baddest monster, use flurry of blows, and stun on every hit. There's a really good chance they'll stun the monster, and they probably spend nearly all of their ki in the process. That's great for the party, but terrible for the monk.
So here's the brew:
Monks can only use stunning strike once per turn, but when they use a ki point it's free for the remainder of that turn.
This paradoxically incentivises monks to use more ki features while spending less ki. They'll be thinking ahead, always trying to find a way to spend exactly one ki point per turn to optimise their chances of stunning an enemy. And they won't be spending it on a boring feature like stunning strike - no they'll be dashing, disengaging, jumping, and dodging all over the place. Because that's how a monk should be played.