You know what, Biden should go on TV and say "if the Court is going to be reviewing so many administrative regulations, it's going to need to be a whole lot larger. Going to need *puts finger in the air* idk, 5 or 6 justices"
IIRC FDR curbed an activist court with just the threat of packing and also there's the fact that when the Supreme Court was revised to 9 Justices it was because at the time there was 9 Circuits. There's more now and a simple rationale to argue increasing it.