sorry for the late reply, and I really appreciate our help. I don't know for sure, but it seems to reset right before uploading. I burned a blink sketch + the bootlader together onto the 644P through via ISP, and the blinking stops for a brief moment and starts again right before uploading. That's also when STK500 errors show up. It's confusing because the program stops, meaning it's resetting, but it starts again, which I'm pretty sure isn't supposed to be happening when entering programming mode. So I don't think the 644P is resetting properly, which is where my problem likely lies. No idea how to fix it tho, since I think my reset circuit is done correctly. I've gone over it multiple times. Also, I'm pretty sure RX/TX lines are soldered correctly. CH340G's RX goes to the Atmega644p'S TX, and vice versa, right?