I don't see how Alito and Thomas show a trend here. Roberts joined after Thomas and Gorsuch joined after Alito, and both have a more independent record.

Among the Democrat appointees, Kagan has a more independent record than Sotomayor but came afterward.

I know there are counterexamples but my point is that there's not an obvious trend toward more partisan justices, or at least no evidence has been presented so far in this thread.