Move/backup cache to 2 locations

Hi yall! 

I've had this project on hold for a while and made an overly complicated mess for myself. 

So, I have an unraid server with an HDD array (soon to be zfs pool after update 7) with raidz1 cache and ECC ram. 

I have another machine with raidz2 ssd's. 

I was planning on putting Nextcloud or similar for a photo library for my family, but I'd need it to be faster than the HDD array and I'd like it to be on the ECC machine for safety. 

I'll take offline backups to an external disk, but I'd also want a daily backup to the HDD array.

The very complicated method to this I've come up with is like so:

Have nextcloud installed on unraid. Write all the new data to cache. Somehow back it up to the Array and move it to the second machine when full so it will read older files from the SSD machine, but not the HDD array.

Be it with the mover or some other way, but I dont want the HDD's to spin up all the time and I want to make sure that I have the files backed up to the array before they leave the cache. 

The only problem is that I haven't the faintest clue on how to implement this, if this is even possible and if this solution is even the best. 

Of course, if someone has a better idea on how to fix or circumvent this issue I'm all ears. 

PS: I dont have any more room in my license for more drives and I'd rather not buy anything expensive just to get to use ECC ram for the photo library.