I think you need a refresher course in the subject of What's Important. As nobody has lived a pristine political life, even Bernie "A rating from the NRA" Sanders, we can't use that as a litmus test. If Lizzy Warren is chummy with Good Time Charlie Baker in Mass, or Bernie is cool with Phil Scott in VT, even though they're both Republicans, I can live with that. As all realistic Dem candidates are in alignment on policy, our goal is to see who connects best with the American people to drive turnout coupled with who can best deflect attacks from the Trump/Putin machine. If that's Bernie, great. If it's Beto, great. If its Biden, great. If we're sick of names that start with B and its Harris, so B it.
However, one thing that will not sell is applying one standard to your candidate and another to the primary rivals. If Bernie's all about full disclosure that's great. Then he needs to release his tax returns as should everybody else in the race.