An update, finally got it working!
I spent some hours trying to remove the BIOS/boot screen on the card, complicated by my motherboard being too new to use freeDOS on USB. I decided to stop trying.
So in the end I emailed Art of Server to ask advice and he kindly provided information on what to do.
It was taping over pins B5&6 and disabling CSM in the motherboard BIOS. That allowed the server to boot into the motherboard BIOS however it would not boot unRAID, so I needed to rename a folder on the unRAID USB to allow UEFI unRAID booting:
I cannot say if taping the pins was needed or if disabling CSM alone was enough - i'm not gonna take the tape off as it took too long to cut the correct size!
Glad to finally be able to finish building my server. 😊