As for migration, you might be able to create a degraded pool initially, copy over the data, and add the parity disk last.
I actually asked in the TrueNAS forum about this idea. According to some knowledgeable users this might work. For anyone interested, details here. The next major release (planned for end of October), should make this easier.
According to this discussion, you can apply your Wayland settings to SDDM from the system settings.
Edit: Although some users reporting issues with it, but worth a try.