Without firmware update 990 is a time bomb
https://www.tomshardware.com/news/samsung-990-pro-firmware-update-released-ssd-health
Run iozone to test pool.
Small tests will reside in ARC and give you false high. 512gb is minimum, or change tunable to reduce arc size
Iperf to test network.
FYI Scale has a bug limiting ARC to half your physical ram. Apparently there is beta test for fix posted on TrueNas forum. And should be in production in 6 months-ish. This won't impact synthetic tests, but in real life you may want to have the extra cache
Free File Sync can auto run when a sd.card is inserted. But you are probably going to want to give a unique folder name for each disk not sure how to do that.
I use a robocopy script to inject photos. Simply enter the drive letter and what you want the folder called.
I all likelihood having 6 SD cards reading at once is going to be slower overall than doing 3 batches of 2. USB kind of sucks that way.
Unless you are going to put 6 dedicated USB hosts in your PC.
This chart mirrors what I have felt, that HDD size increase has slowed way down.
We had a 500 mbyte in our win 3.11 box in 94. 2004 I think I had an 80gb