Expanding the high court would be going down a rabbit hole. We would expand, then they would expand, and soon we'd have as many high court justices as we have members of congress. And WHO wants THAT ????
No, what is needed are term limits for justices.
Either that, or do like they do for state court justices where every justice comes up for a retention vote. Have US supreme court justices come up for retention vote, oh say every 4 or 8 years during a national presidential election cycle, and let the people decide. And should any justice be rejected by the voters, the "president elect" would be allowed to fill and replace justices who the voters rejected.
Seriously, there must be better alternatives than JUSTICES FOR LIFE. In fact, term limits should be also be applied to EVERY elected politician both house and senate. We already have term limits for our presidents to avoid gaining too much power, well.... with politicians like Mitch McConnell and Chuck Grassley we need term limits there too because too many people like McConnell and Grassley already have an obvious power problem. Considering what McConnell pulled on Obama but refuses to apply to Trump, it is clear that McConnell has more power than a president. Term limits up and down the line IS THE ONLY ANSWER. That includes term limits on high court justices as well. Term limits up and down the line WOULD solve ALL of the problems with power hungry politicians. Power hungry politicians "are" the main threat to our democracy.