That system is plenty capable, in fact you could drop out the GPU if you don’t have anything that will specifically be using it. It’s just going to draw power when all you need is a video-out port. The qty of RAM is particularly good, will provide plenty of cache for the array and the CPU is strong enough you could enable full-drive encryption and not have it be a bottleneck (assuming that is something you want).
TrueNAS’ boot drive doesn’t need much space or speed, might as well boot from the 250GB SSD and save the NVME SSD for w/e else you want.
For array-type, a RAID 5 (or ZFS equivalent) is going to be your best setup. Will use one drive as parity and the other three for storage. A single drive failure is allowed with no data loss.