So with the case/mobo/power supply at $259, the CPU/GPU at $329, you’ve got $11 left to work with to buy RAM and SSD, in order to be competitive with the base model Mac Mini.

That’s what I mean. If you’re gonna come close to competing with the entry level price of the Mac Mini (to say nothing of frequent sales/offers/coupons that Best Buy, Amazon, B&H, and Costco run), you’ll have to sacrifice and use a significantly lower-tier CPU. Maybe you’d rather have more RAM/storage and are OK with that lower performing CPU, and twice the power consumption (around 65W rather than 30W), but at that point you’re basically comparing a different machine.