They don’t have to expand the court, all they have to do is institute a term limit of 10 years — instantly Thomas, Alito, and Roberts are off the court and the Dems can appoint replacements that make the court far left for a minimum of 10 years.
Retroactive term limits would be struck down by that same court. They would also lose some old guy, forgot his name, and Roberts needs to go anyway.
Term limits requires a constitutional amendment. A simple law will not accomplish anything. Article III, Section 1.