Of course, the reason Mueller can’t or probably won’t testify to congress is because there is a Federal criminal investigation into the origins of the Trump/Russia investigation. U.S. Attorney John Durham is the prosecuting attorney. I’m sure some of the Democrats in the House of Representatives would love to get a hold of some of the witnesses.
And, of course, it would be a felony to illegally procure Trump’s tax returns, even if it COULD be done. But if there were any shady dealings going on, the IRS would have uncovered it. People like Trump are constantly under federal audit
Unless there is actual evidence of a crime, even good circumstantial evidence, any speculation is really just noise.