Welcome to the forums
@Krackster - I appreciate the detail in the hardware configuration and photos (and applaud the creative concentration of 18x 2.5" SSDs into a compact form factor!)
If you sign into the TrueNAS SCALE web UI, is your pool visible under the
Storage menu, and is the iSCSI service running under
System -> Services?
I do have some slight concerns about the M.2 expander cards - I assume these are the ASM1166-based expander cards I've seen, which are better than the JMicron ones in that they don't use a SATA port multiplier, but still not as well-supported as the traditional LSI option.