We have 2 SG230 with both eth0 up to eth5 (HArdware)
However - we can only assing 5 ethernet interfaces to acutal interfaces (eg WAN, DMZ, internal, external, test).
If I try to create another interface - the "Hardware" selection is empty.
This is the same for both SG230 we have (not in HA of course, both standalone)
you just remove the "Sync NIC" from here and deactivate HA, then (Operation mode to "Off"):
This should should free up the missing Ethernet interface for you.