I think this is probably the closest I’ve seen to a feasible answer on the above.
Mueller won’t be expecting this Senate to impeach Trump and we are half way through this presidency already anyway. If he can’t go after Trump directly, he can definitely go after Trump Corp and all it’s filthy minions which is what he’s been doing.
He can start dismantling it and charging those guilty of money laundering, freeze accounts and basically cripple the Trump criminal empire. That process isn’t going to be done in 2 years so when Trump is a private citizen again, he will be fair game for subpoena’s and perjury charges.
As far as the GOP go, if they don’t impeach despite good cause, it will surely be very difficult to support him as a candidate for another term. Especially if there are investigations and prosecutions still ongoing.