I'm thinking maybe Ceph with a tiered cache setup of SSDs or 10k/15k drives in front of a archive drive pool with erasure coding(raid5) could work for backups. This essentially lets you put a SSD in front of the drives that is used both as read and writeback cache, so writes would complete very fast and slowly get moved over to the shingled drives over time.
Doubting the usability for video editing and the like though, even if the files on first read gets pulled into the cache drives, it'd still be *really* slow on first access, in my experience parity raid isn't exactly suitable if one needs crazy good performance except in some rare cases(And in the case of ceph, it's slightly worse due to the block size being 4MB by default, which gets read into the cache disk if even a 4k sector is read).
TL;DR: I have read up way too much on storage
