LVM Ontop of LUKS using Grub

Your problem seems to be in the difference of :crypt as volume group for /dev/sdb2 and using lvmpool- as volumegroup name as parameter for root.

GRUB_CMDLINE_LINUX="root=/dev/mapper/lvmpool-root cryptdevice=/dev/sdb2:crypt ro"

The example here:

cryptdevice=/dev/partition:MyStorage root=/dev/mapper/MyStorage-rootvol

has matching :MyStorage and MyStorage-. That page specifically targets grub (and not Syslinux), with LVM on top of LUKS. So I would follow that set up.

That you have an extra, not encrypted partition, doesn't matter.