Keep root file system from external HD?

I am using a Raspberry Pi 3B+. Until now my FreedomBox was booting from one USB drive while having my root file system on another (larger) USB drive. (To replace the root file system I followed the thread here:

Although this set up generally worked, it had problems when rebooting. I do not understand what the problems were, but I assume it had to do with booting from the USB drive somehow. Typically after a few tries of turning on and off the Raspberry Pi it would eventually reboot properly and my FreedomBox would be back online. After the last reboot yesterday, however, it seems the server is offline for good.

So I want to switch to a system in which I boot from an sdcard but still have my root file system on the same larger USB drive.

My question: Is it possible to flash a new sdcard and keep the old root file system from the previous set up? Might something like this work after booting the newly written sdcard?

In FreedomBox:

$ btrfs device add /dev/
$ btrfs device remove /dev/mmcblk0p3