I simply take the final sequence, and Consolidate it to an external drive. And then archive the entire project there as well. I have the bin that is the consolidated cut, and keep all the other bins just in case we need to recapture anything.
I keep a copy of that project on my self, and give the client a copy as well...adding it to their bill.
I'm sure it's assumed, but all external graphics files, and camera/audio originals are also backed up. You seem to have the other assets all taken care of. When it comes to backup and archive it becomes a decision of short term, mid-term and long term storage. I don't really trust long term storage to local drives. I assume those are my short term solution (3 years max with some TLC every 6 months) and those are double copies in two different locations. For longer than that, I make an LTO backup. For a mix of long term is Amazon's Glacier. Their redundancy and maintenance is awesome so there is no maintenance on my end. There are several GUI's out there for accessing the archive such as: http://www.cloudberrylab.com/ppc/new/backup-to-amazon-s3.html?gclid=CKWc-P3...
And the cost is reasonable on a per program basis. For example, a feature I did all mastered as DNxHD 175, and all audio deliverables, project, trailers, is under 150GB. For that price, it is $0.15/month. It is more expensive to retrieve (like 10x factor), but still quite cheap when pulling from the Glacier archive is really because the drive and the LTO are destroyed.
Please Please Please, do NOT rely on spinning disks. If you use hard drives in the short term you should have the files in at least three places, one of which is in a location separate from your editing computer. The Academy and others have white papers on proper archiving and the procedures of some film libraries.
I am surprised that some folks that a notalentwannabe like myself looks to for guidance are not on top of this issue. We stand to loose many movies.