I think they go for the guys around Trump before they do anything towards Trump himself. They know he'll throw anyone and everyone not named Ivanka under the bus if he thinks it means he's getting away with it.
This investigation is like boiling a frog. Heat things up and the frog jumps out. Heat things up slowly bit by bit and he gets use to it and boils.
They'll heat things up on Trump as slowly as possible... but I still think it's well before the midterms, maybe by fall. We could get Stone and Cohen soon. He'll rant and rave and freak out. They'll let him get use to it.. then go after a few more higher level guys (Kushner, etc). He'll rant and rave again then when he calms again if they have the evidence they'll go for him. I think maybe by September or October.