I'm fairly sure of this (I've never dealt with the encrypt stuff) as I am a disliker of Plymouth.
Step one is to uninstall plymouth.
Then disable sddm-plymouth - from a console, not a terminal app. Stopping sddm will also halt your desktop.
systemctl stop sddm-plymouth.service
systemctl disable sddm-plymouth.service
and enable sddm,
systemctl enable sddm.service
and on reboot plymouth should be gone.
You may want to remove 'quiet' from your kernel parameters (if it's there).
HTH