yep you could move rootFS as well as bootFS to USB/SSD devices
Maybe you can have a look to the following forum post. There are already a couple of descriptions how to transfer from SD to USB devices
https://dietpi.com/forum/t/services-failing-after-swapping-backup-drives/3869/18
https://dietpi.com/forum/t/services-failing-after-swapping-backup-drives/3869/6
https://dietpi.com/forum/t/move-boot-partition-to-smaller-sd-card/3687/7
as well it should be possible to flash DietPi directly on an USB device right from the beginning. But honestly I never tested this 