Hello singhraj,
Disk-Groups require exactly 1 SSD as Cache-tier device and up to 7 HDDS/SSDs as Capacity-tier devices - you shouldn't attempt to mix and match Hybrid and All-Flash Disk-Groups either.
So yes, 4 Disk-Groups per node with 1 SSD + 1 HDD per host or 2 Disk-Groups with 1 SSD + 2 HDDs per host would be valid options here.
I would go with Witness over NUC if the other nodes are actual servers as the performance disparity between an actual server and a NUC would likely be large aand thus it will drag down the overall performance and/or stability. As this appears to be a home-lab/test setup you could also run the Witness on the NUC.
Bob