For these you can make the volumes smaller than the SSD and grow them on the fly pretty easily later on as you need more space in home root or var.
vghdd mapped to md2
storage lv on vghdd mounted to /storage
You could format all the data logical volumes btrfs2 if you'd like and encrypt with LUKS! That's the Synology recommended config, I've also used xfs with my important volumes with no issues.