I'm doing essentially the same thing as you. I've found that having all partitions under 2Tb helps a little. I would delete your partitions and recreate them, making sure they are under 2Tb. You can then create B2D folders for each partition and add them all to a storage pool. Point your backups to the pool and BE should work OK.
My backups run, it's just that they are averaging about 270/Mbps. I NEVER have a successful backup within the 12 hour window I have. Also, make sure that when you setup the B2D folders, you change the "Maximum size for backup to disk files" to whatever your partition size is. If you don't, you'll end up with multiple 1Gb .BKF files. That will slow things down even more.