Let's just be clear - Julien hasn't proven anything yet. Pitching in the Arizona Fall League is lousy this season. I would like to see a half season at AAA before he debuts.
They can afford to move Polanco because they have another All-Star 2B in Arraez on the roster as well as plenty of MLB caliber depth (Gordon, Lewis, Miranda) and several interesting prospects (Martin, Lee, Julien, Schobel, Miller and the surprising Dalton Shuffield) in the infield.
The upcoming free agent market is actually pretty lousy for 2B. Jean Segura and Brandon Drury are the top talent. Both are older and less desirable than Polanco.
I wouldn't even target anything in particular for Polanco. Get the best overall talent you can find. If that includes a catcher even better.
The reason NOT to move Polanco is he is coming off a down year with bad defense and time lost to injuries, If they can't find a good return he can absolutely help this team in 2023.