why do I need to partition the drive and why do I only see the last partion?
good questions - I noticed the same here.
I noticed that ESXi only sees iSCSI targets that are partitioned.
Sorry I have no answers but I think you are lucky that it did not work.
IMHO the LUNs you are planning to use are way to large for a VMFS volume.
In my recovery practice I regularly have to dump full LUNs to have a backup in case the repair goes wrong.
In this respect LUNs over a few TB would prevent recovery attempts as you would need days to dump it.
Cant you create several smaller LUNs with your RAIDcontroller instead ?