Bad news for you:
iSCSI works best with mirrors and less than 50% occupancy.
If you want to use raidz2 for space efficiency go for SMB or NFS. Then you might safely fill your pool up to around 80%, tough at some point performance will suffer; then around 90% performance will go down the drain as ZFS switches to a different allocator, and you should never ever reach 100% (=locked pool!).