"PCIE5 slot (
BTO)" Built-To-Order! You have to specifically order a board with this slot mounted to experience the limitation. As pictured, there's only an empty green space behind the M.2 socket where SLOT5 would be mounted.
Otherwise, a server board like this is just a regular Z490, but optimised for 24/7 operation, no overclocking and no care for "aesthetics" because it's bound to be locked away in a server room or a far-away data centre rather than adorn a fancy case with tempered glass panels.
If you don't mind the screw-in, finicky, drive trays (I certainly would!), the Define 7 is just fine for any "server" board that fits in.
The issue is more about reorganising drives—and, if possible, considering a safer raidz2 layout because raidz1 is actually not guaranteed to sustain the loss of one drive when using such large capacities. How much data is in there?
Read cache (L2ARC) would only be useful if you were repeatedly streming the same files over and over. I doubt that's a realistic use case. And then, L2ARC takes up some ARC space (i.e. RAM); with only 16 GB, a large L2ARC is actually hurting performance.
Best take out the cache, and keep two NVMe for boot and jail — separate, as wanted, or hacked into dual use
Moderator Warning: This HOWTO will damage the ability of your TrueNAS appliance to properly maintain itself, will break the ability to replace failed drives, and may cause problems with upgrades and updates. TrueNAS is not designed to do this sort of partitioning, and the developers are not...
www.truenas.com
That will free up a useful SATA port for the data pool.