I think it's going to happen as 100% a pander to the base, many of whom are frothing at the mouth for it. It'd get them off the hook all "Hey, we tried!"
This is correct. Mueller will likely offer a report that enough grassroots Democratic voters will read as being incredibly damning that, presumably, Dems will pretty much have to hold impeachment hearings just to satisfy their own voters. If they get the Mueller report and then Democratic leadership in Congress says "No impeachment. We'll just hold him accountable at the next election.", that just isn't going to fly with the party's own voters. Doesn't matter that conviction in the Senate is essentially impossible since Republicans won't back it.
What's more interesting to me is the possibility of Mueller issuing a report just on the obstruction of justice side of things first, as reported by the WaPo:
https://uselectionatlas.org/FORUM/index.php?topic=286476.msg6257858#msg6257858If that happens, then do the Dems in Congress advocate patience, and wait for Mueller Report, Part 2 to come out? Or will their voters already be pushing them hard to launch impeachment hearings, just on the obstruction of justice charges alone?