Because I have a significant number of large files which do not change (reference only), and "archive" type files (finished projects) which I retain only for future reference, I have a "two-backup" strategy, consisting of (1) a Time Machine backup for my boot drive and all my applications and active work, and (2) an "Archive" drive, cloned automatically on a periodic basis, so all that archive-type data is available for reference but is not taking up space in my TM backup.
If I had a setup like yours, I would use the external as you indicate, for your iTunes library, and other things of an "archival" character, and automatically clone that (using Carbon Copy Cloner or SuperDuper, etc.) to one of the RAID drives. Then I would use the other RAID drive as a TM backup for my startup drive.
It is helpful to remember that Time Machine backs up and keeps every version of each file, so the more files you are backing up with TM, the faster it will fill up your TM backup drive. To my way of thinking, files that are "reference-only," and are not going to change, do not need to take up space in a TM backup. I have a 500GB Time Capsule which has more than a year's worth of TM backups of the roughly 250GB of data (system, apps and documents) on my boot drive.
You might consider something similar. Hope this helps.
