A quick search shows 2 Intel motherboards with mini-ITX form. There was also a micro-ATX.
https://download.asrock.com/Download/e-catalog/ASRockRackPro...
I've also bought from Gigabyte before. The selection isn't as good, but I see an AMD and 3 Intel with mini-ITX. You might have to get separate embedded versions of the processors.
https://www.gigabyte.com/Enterprise/Server-Motherboard
I think Intel has more options with smaller boards here. Always check the block diagram to see if the IO matches how you'll use it.
I got this one for $600:
https://www.amazon.com/gp/product/B08FB5LBJP
If you outgrow a 9-bay NAS, they also sell full storage servers with management.
I was very tempted to build a NAS too, but ultimately prebuilt was better I think.