“Do you think they could get by with passing a law expanding SCOTUS, which at the same time prohibits any future changes to its size?”
No, they cannot do that without a Constitutional amendment, nor can they require a super-majority to change the law. It is well-established law that a current Congress cannot bind a subsequent Congress from repealing or altering a statute.
That's the problem. Starting with the ultra-liberal Warren Court, many "well-established laws" were thrown under the bus.
Since the GOP's only short-term counter-move in the 1950s required an impossibly high threshold of 67 votes to impeach and remove, they just sat helplessly and watched one law after another being enacted by 9 unelected, life-termed men.
Are you absolutely confident that a 19-member SCOTUS with a Millennial Marxist majority a year from now will abide by the "well-established law" conventional wisdom?