Whoop, it works, thanks! I've followed the (non-boot) parts in your tutorial. thus I created a single partition on the SSD, formatted it with LUKS/ext4 and copied the rootfs over to my SSD. I've performed some steps on the original (source) OS as well, such as installing dropbear, copying the authorized keys and editing the crypttab and fstab (basically step 9) + update initramfs.
After rebooting and configuring the new rootfs such as providing new root password etc. I've added two lines to the fstab on the new or target-rootfs:
UUID=old-rootfs-uuid-here /oldfs ext4 defaults,noatime,nodiratime,commit=600,errors=remount-ro 0 2
/oldfs/boot /boot none bind 0 0
Now the new system is aware of the boot folder. When I run update-initramfs on the new rootfs, the boot folder is updated accordingly. Not sure if this is the best way to do it, suggestions and improvements are welcome.
When starting the newly copied system for the first time, I noticed that I couldn't use ssh to login for the first time (with root + password "1234"). Is this expected behaviour?