The encrypted volume can be successfully decrypted, so at least you should
have no problem to make a backup.
If you want to set up full-disk encryption, it is strongly recommended that
you perform a new installation and "erase" the disk (filling with random
data).
During installation, re-create the partition table on the destination disk.
Create a non-encrypted partition for /boot (and one for /boot/efi, for
UEFI/TianoCore payload). Create an encrypted volume using all remaining disk
space.
After setting up the password (and random filling) for the encrypted volume,
you should create physical volumes (PV) on the encrypted volume, then create
volume groups (VG) on the PV, and finally create logical volumes (LV) on the
VG.
Once you have configured LVs, you can assign them to different mounting
points like /, /home, etc., as well as swap area. Do not create swap area
outside the encrypted volume, because this would defeat any security provided
by full-disk encryption.
Configuration of full-disk encryption using Debian Installer is quite
straightforward. A professional installer should be like this.
