You all act like Mueller was the one who convened the SC and was the one doing the investigating.
The SC was called for by the Democrats. It was empaneled and then RR put Mueller over it.
The SC was the group largely responsible for the report and its content. Mueller was only able to place it in context.
Mueller would not determine whether the Dossier was fake/phony or not. That is for a court to decide. Always has been. However, you can't have parallel construction. IE - you can't have two investigations of the same exact thing or build two legal cases against the same crime.
So, Mueller's investigation shut down any attempts by the Democrats to launch additional investigations and kept all parties on it largely under control.
Mueller's testimony passes the buck to Congress. He did not want to testify for optics. When Congress dragged him in to testify - he basically said "if you all want to read the report and say that the President committed obstruction, you have the power to do so. Whether I think there was or not is irrelevant. You just want me on record so that you can say you are acting on my judgment. I am giving you no such out. Own your allegations."
Of course the SC was going to be biased against the President - all grand juries are biased against the defendant and required to be so, as a person who is innocent of an apparent crime needs to have the trial settled in court.