you seem to be forgetting two things.
Most crucially, the Mazar’s case involved private financial records. Trump could not, and did not, bother to make any executive privilege claims. The courts have recognized that a President has very broad executive privilege authority – and have given Congress access to documents despite those claims only because of highly specific circumstances that do not apply in this case (see Oversight v Holder). The Mehta opinion in no way impacts the executive privilege doctrine, and how it generally overrides congress’ “oversight” functions. But US v Nixon said that “judicial proceedings” can trump executive privilege claims when evidence of criminality is being sought. As long as this is just about “oversight”, its unlikely that the courts will overturn “executive privilege” concerns — otherwise, all that congress has to do is say “well, maybe there are impeachable offenses here” and executive privilege goes “poof”.
additionally, by invoking impeachment, its far likelier that the courts will provide expedited review of executive privilege cases. The Holder case took OVER THREE YEARS from the first subpoena to the first document was produced.
No one here ever denied that Congress could “investigate” whatever it wanted to – or that a case like Mazars was a slam dunk for Congress because all Congress needed was a legislative purpose to compel private entities to provide documents.