System Pool size and ZFS (QuTS) RAID 5 expansion

[desless1]

Hi,

I have a TS-h973AX NAS, which I was thinking of using 2x 120 GB SSD (RAID 1) as System Pool and 2x 500GB (RAID 1) as Read cache.
My question is will the 120 GB size be enough for system pool? My understanding is that most of the apps and such + home / public folder goes there.

Also I have read that ZFS and QuTS does not support to add another drive to i.e., RAID 5 to expand, is that correct? If its not possible, will it be in the future? Or is there some technical limitations that will not be able to implement that at a later stage?
If so I am tempted to go for QTS, on the device just to be able to expand with disks at a later stage. Thoughts? Or ideas on how to mitigate that?

Thanks!