The court will likely say, on a 6 to 3 vote, that Presidents have immunity from criminal prosecution for their official and unofficial acts which fall in the outer perimeter of their duties. Then, I suspect they will kick the case back to lower courts for determination on what constitutes official and unofficial acts. Depending on the lower courts decision, it may then make it back to SCOTUS for a ruling. Highly unlikely that the Jan 6 "case" will make it to trial prior to the election. If it did, it would only strengthen Trump's poll numbers further.
Presidents must have immunity for official and unofficial acts during their presidency otherwise they will be forever concerned about retribution and this will affect their ability to govern effectively. This includes presidents from all parties. Had Trump done something illegal, I would be the first one to say he should have been impeached, convicted and prosecuted.