if I personally save all my wallet.dat files like 10 different USB sticks offline. I have done this since I first started Bitcoin. This is the best choice for me. Simply time-consuming updating of each wallet file because I have so much and they need to be stored in all the sticks again. Better to be safe than sorry. After our missing data do not happen
10!!!
Firstly USB sticks are Inexpensive devices which can be very good for backups for only for a short time because I don't trust them for long-term backup especially if the USB is not plugged in PC for a long time as there is a high risk this/these device(s) can lose their data including the wallet.dat file(s)
As you are updating often them one by one then the reason above^^ doesn't worth much, but what I would suggest you is to encrypt these files when you store in the USB, and probably storing and somewhere else like the hardware wallets which are not costly.
I bought my Ledger Hardware Wallet HW.1 very cheap like $20 at that time probably and it works just perfectly.