Well, I finally got a properly functioning computer up and running. I returned the prebuilt one because it was regularly throwing up stop errors from the hardware abstraction layer. I ended up building this thing:
https://pcpartpicker.com/list/shkXHB; for about $225 more than the prebuilt, but at least now I know what the power supply is, anyway. It's pretty much the same thing as the build I was originally thinking of; I just went with a different brand of memory, decided to stick with the stock cooler, took the GPU up a notch to the 2070 Super, went with a more efficient power supply, and cut back on the MB with a much cheaper X470. Unfortunately, the MB didn't have a new enough BIOS for the third gen Ryzen. So I had a few options: Take it to a local repair shop for a fee; Send it to MSI for the cost of shipping; Get a bootkit from AMD. I went with the most free option of getting the bootkit from AMD, but not before getting an RMA number from MSI. I wasn't sure AMD would actually send me the bootkit, though, because MSI had technically offered to do the BIOS update. Fortunately they accepted my reasoning for why it wasn't a 'satisfactory' solution. Anyway, they sent me an Athlon APU to flash the BIOS with, and the rest is history, as they say. It cost me a lot in idle time, but it was pretty painless once I actually got the bootkit. Unfortunately, I also realized that the PSU I got is like a mini-ATX power supply, or something, and the CPU/ATX power cables are comically short for an ATX case/MB. So I had to get some extension cables, too, which kind of sucked.
Anyway, the case feels kind of cheap, and so does the rear I/O on the motherboard, but so far I'm pretty happy with it. It's pretty nice and quiet, even under load, which is something that was pretty important to me. It's also running way cooler than the prebuilt (like 20 - 30 degrees C, cooler), which is nice. I don't know if that's because of the BIOS, or the Arctic thermal paste, or just the difference in ambient temperatures over the last couple months, but I'm happy about it. I did a fairly abbreviated stress test on the CPU, but the temperature topped out at 70 C. I'm really only targeting 1080p @ 60 Hz right now, but it's pretty much like, 'Metro Exodus? Maxed out quality settings? Ray tracing on high? *Yawn* Yeah, whatevs. Here you go.' Just the other day I realized that Tetris Effect is on the Epic Store, so now I have to debate whether or not I want to get a VR headset right away--.