Ya, sorry but no. Putting backups on the cache isn't a solution.
The fix is using MariaDB and having it on your cache.
Close but not quiet.
Just have NextCloud and MariaDB on your cache and that's all you need. You can leave your Backups share on the array.
I was just using sqlite which was accessing the owncloud.db file and keeping my disks spun up so I changed it to MariaDB which needs to be on the cache along with NextCloud. Once I did that my disks would spin down and stay that way until normally accessed again from whatever gives them a poke. My disks have been down for a couple hours now and i'm quite happy about it.
Check out >Spaceinvader One's Video< about setting up NextCloud with MariaDB. Just follow the instructions and you'll be good to go. If you don't have your appdata on your cache though make sure to manually change the path to your cache drive for the dockers.