In case you’re unaware, the way to boot from a different drive is to do the “smartphone recovery boot” of pressing the power button and volume down button at the same time while the Steam Deck is off. It will boot to a drive selection screen which you can choose which drive to boot from.
In other words, you should just be able to plug in the Tails OS USB and boot from it using that button-combination. Cheers, and I hope it works out for you!
EDIT: Also, since your username is “Qual” here is an obligatory James Quall animated gif: