I think what you are asking is fairly simple using the custom partition setup in Calamares
- remove all partitions
Create
- an efi partition fat32 - 512 M - boot flag - mount on /boot/efi
- a boot partition - 1G - ext4 - no flag - mount on /boot
- a root partition - the rest of the space - root flag - mount on / - encrypt the partition
This will put your kernels on unencrypted space and the rest encrypted - no need to mogrify to sd-boot.
Yes - previously the decryption phase was a long wait - I think this has been resolved with the latest iteration of the grub boot loader.
On my playground - I installed a system yesterday using btrfs - swap and hibernate using full disk encryption - and yes the unlocking takes a little time but that is expected and it is nothing compared to what I remember.
There was one small caveat with my test though - which has been reported