I don't think Mueller is worried about building a foundation for impeachment.
And at this point I don't think the House should impeach. Maybe they will feel obliged to, but the political risks are large. Endless "investigation" and testimony are just going to distract from all the new stupid **** Trump will do, and probably energize his base. There's no real hope the Senate will remove him from office and even if it did, President Pence.
The Senate IMO is not going to worry about campaign violations or obstruction of justice, but Trump's ongoing threat to national security might flip a few of them. Maybe that damage has to happen to open some more eyes.
I don't understand why Trump nominated William Barr as AG. In his confirmation hearings Lindsey Graham has vowed to protect the Mueller investigation and Barr has indicated he won't interfere, yet somehow Trump is convinced Barr is a rubber stamp with personal loyalty to Trump. Who is advising him? Is Barr only pretending to play it straight?