Be Quiet! Dark Rock TF came in. I just finished removing the stock cooler and installing the Be Quiet! HSF with some fresh Noctua NT-H1 thermal paste on the 8700.
Idle temp is now down to 26C, and the temp running AIDA64 system stability test (stress: CPU, FPU, cache, system memory) for a few minutes has gone down from throttling (~100C) to topping out at 76C (highest temp of any core peaked at 82C). 100% utilization of all 6 cores at 4.3GHz throughout. Much better results than the stock cooler (no surprise there) with virtually silent fans, even up close in open-air.
Price was steep for an air cooler at $80 on Amazon (resellers on Newegg want even more at $120) which is encroaching on midrange AIO liquid coolers. It's worth it for me for the silence, and the fact that this build is going in a custom 3D printed case (not finished yet) that won't have room for an AIO unit.
I don't think I would recommend the Dark Rock TF for anyone using this motherboard (ASRock Z370M-ITX/ac) as it doesn't quite fit properly. The heatpipes are pressing against a heatsink on the motherboard (VRM?) on one side - enough to bend it - and touching the first low-profile stick of RAM on the other side. It would likely be a much better fit on a micro ATX motherboard, however.
This video was really helpful, by the way, as there were one or two confusing steps poorly outlined in the installation guide. Otherwise it was about the same as any large single or dual tower heatsink installation, requiring the use of a backplate.
Also updated Windows with the Fall Creator's Update (aka version 1709) and everything seems to be working okay. Haven't tested to see if it affects gaming, as I still need to get a GPU. I'm waiting for the 1070 Ti, though I may end up getting a mini regular 1070 or 1060 instead.