Hi Andreas,
2. delete all Partions on SD-CardIHMO there is no need to do so. Just keep them as a rescue system.
3. dd CentOS-Userland-7-armv7hl-Minimal-1511-CubieTruck.img.xz to SSDInstead of dd'ing you can copy the filesystem contents to your SSD and choose a partitioning layout which fits better to your needs. This wiki article ( http://bit.ly/2gcXepm) addresses Aarch64 but it should work with the cubietruck image as well.
I don't know the boot loader capabilities in detail. One thing you have to change in any case is the command line passed to the kernel (bootargs). If the boot loader is able to load from SATA/USB, you can place the /boot partition to the SSD (and modify the boot loader commands to load the kernel image, etc.).
4. modify Boot-Configuration to to have Bootloader on SD-Card ant the OS on SSD