Impeachment is a high risk strategy for the Dems. They won't get him impeached, regardless of what he has done, and he can rattle on about harassment etc.
I still come back to the fact the Dems need a clear and convincing front runner (it's not Biden surely) and some appealing policies or Donny is in for another 4 years and this thread will get to 2000 pages not just 1000. They shouldn't be wasting their time on this.
Once they are in power they can go after him and his whole rotten family on tax, corruption etc in the private sector. Much easier sell to the country I would have thought.