Why should members of Congress be protected from investigation? There were members at the Rally on the Ellipse & there were those who promised to disrupt the certification of the Electoral votes. And even after the Insurrection, senators and congressional reps still objected.
And this doesn’t seem that difficult to deal with. The FBI for the most part should know who breached the Capitol or engaged in criminal acts. If members of Congress communicated with those individuals, it will merit further investigation. If they didn’t, their data will be purged.