It’s viable, but when you’re buying a DAS for the drives, figure out what the USB chipset is and make sure it’s not a flaky piece of crap.
Things have gotten better, but some random manufacturers are still using trash bridge chips and you’ll be in for a bad time. (By which I mean your drives will vanish in the middle of a write, and corrupt themselves.)
One thing you probably need to figure out first: how are the dgpu and igpu connected to each other, and then which ports are connected to which gpu.
Everyone does funky removed with this, and you’ll sometimes have dgpus that require the igpu to do anything, or cases where the internal panel is only hooked up to the igpu (or only the dgpu), and the hdmi and display port and so on can be any damn thing.
So uh, before you get too deep in planning what gets which gpu, you probably need to see if the outputs you need support what you want to do.