@Blameless @Biggu @o1dschoo1
You guys were right! Took the AIO all apart again (Lepa Aquachanger 240) and drained it completely except I filtered it through a coffee filter. Next I ran vinegar though the hoses and radiator and let it sit for 5 min. Let the cold plate and pump housing sit with vinegar on them for 10. Flushed with vinegar, then tap water, then did a bunch of flushing with distilled water. Quite a bit of gunk came out. Put the drained and filtered original liquid back in and topped it up with distilled water and reassembled it all. Installed it back in my tower and it's cooling just fine! Even better than pre-lapping, actually, so in spite of the problems I caused myself, I was still successful 😂
I know that putting in fresh coolant would be best, but the point of the project was to spend as little money as possible so I just put the old coolant in. When this thing dies, I'll probably switch to the Arctic Freezer II.
I think what happened is that when I topped it up with distilled water the first time, I did a LOT of shaking to the hoses and radiator and ran the pump for brief 5 second intervals to get any air bubbles out. I think the combination of shaking it a lot, and running the pump multiple times, cause debris to break loose and then pile up at a single choke-point and that was killing the flow-rate. And of course the low flow-rate meant it's cooling capacity went to ****.
Thanks for the help, everyone!