QNAP TS431P SSD or HDD?

I have a LOT of genealogy photos and videos (~10TB) on my desktop plus my wife and kid's laptops.
I got a free TS-431P from work and bought 4*6TB WD Reds to put in it, but I now see how it MIGHT be faster with an SSD in the first slot.
I mainly want to use this NAS for backup of all of the workstations. My understanding is that I am maxed out at 16TB by the CPU???
I have an external 16TB USB drive for backup of the NAS. I was thinking that I should only use 3 of the HDD and an SSD for speedup?
What is your recommendation?
I am also having an issue figuring out how to schedule the sync with the external USB, could you point me to one of your videos? I watched the 8 part one, but maybe I missed it? THANKS for all of your hard work.

There is no limit on volume size, it supports even 18TB drives. But 1GB RAM is not very helpful indeed. As a backup NAS this is OK, but nothing more really.Adding SSD in a first bay would speed up Qnap apps and general user interface. But the real bottleneck is RAM.Yes, you can automate external USB disk backups. Here is how :https://www.qnap.com/en/how-to/tutorial/article/how-to-use-hybrid-backup-sync-to-backuprestoresynchronize-your-files-on-a-qnap-nas/https://www.qnap.com/en/how-to/tutorial/article/creating-an-external-backup-job/https://www.qnap.com/en/how-to/tutorial/article/how-to-use-hybrid-backup-sync-to-backuprestoresynchronize-your-files-on-a-qnap-nas/#:~:text=To%20backup%20the%20files%20on,start%20creating%20your%20backup%20job.&text=To%20backup%20your%20files%20on,%2C%20and%20click%20%22Apply%22.I hope this helps.