That's a good topic, as I want something custom and have nice mATX board with 10G and SAS3 controllers onboard and three PCIE slots available.
Currently, I quite like CS381, but it's expensive, plus you cannot have two-slot GPU in bottom PCIe (need 5 PCIe slots on back). It's also pretty tall, but that's not a big deal. Everything else is kinda ok for me.
I don't think many people interested in 12x5.25" devices. You would have 8-10 3.5" drives at home. Anything more and it's easier to get rack storage. 8 Drives seems optimal to me, with spare space for a few 2.5" SSD of course
With a typical Hi-Fi equipment being 17" wide, it makes perfect sense to create a case that is 17" wide. CS381 is 15" or so, and extra 1.5" would add space for beefier videocard, and would look better in Hi-Fi rack or next to TV.
Crazy idea #1: mATX case with 8x3.35 drives, that can only take Half-height cards. All PCIE SSD are HHHL (half-height, half-length), lots of HBAs are short, and you can find small videocard too. Make it deeper, and place fans next to drives, and you can get adequate cooling IMHO.
Crazy idea #2: There'are some Intel/HP/Dell cages for sale on ebay with backplanes. 8xSFF drives or 6xLFF drives. It's way way better then istartech, and looks way more professional. Kinda expensive, but still. If there'a case with cut, so you can choose cage of your choice (LFF/SFF/NVME) it would be interesting.