Not your scenario really but a HBA will allow you to use SATA and SAS drives. Gives a bit more flexibility on price, especially with 2nd hand SAS drives.

My drives are currently in a box supplied by wires hanging out of the PC (Server!) casing but it would look much neater with a 4 bay hot swap cage built for the purpose.

I just wish I could get the full 12 Gb/s out of the couple of SAS drives I use :o(

You may be out of luck, but doesn’t that board have onboard VGA graphics which you can use?

OMV is a web page dashboard and there’s SSH. What is the usecase for a GPU I wondered?