drop back and punt with my old i5-4590 tower

This one. It’s old, but with RAM and a GPU you’ll be able to handle streaming/transcoding.

The problem with Dell/HP/Lenovo etc. PCs, especially smaller form factors, is that they’re non-standard, as you found. And not just the motherboard mounts, but also the power supply connectors, and often other stuff too. And any feature found on a typical board, but not used in that particular model, will be missing, like multiple SATA ports, again as you found.

You could probably get a newer desktop for not much money, too.