Or is it enough to disable
"Hardware TCP Segmentation Offloading"
"Hardware Large Receive Offloading"
Those should be disabled anyway, they are disabled by default so definitely disabled them if you have set them enabled.
Hardware offloading requires the driver and hardware to work correctly together. Something that works on an igb NIC might work on ix. It might not even work on a different NIC that also uses the igb driver.
They usually do though because those Intels are the best supported. Intel contributes their own driver code to FreeBSD.
To disable that as a test you can run at the command line:ifconfig ix0 -vlanhwfilter -vlanmtu -vlanhwtag -vlanhwcsum
I had assumed your igb NICs are not SFP?