If you get that water up to 80C I guarantee you the CPU and GPU core will be a fair bit hotter than that. The real answer is that it's much more practical for most people to have an efficient system that can keep up with the thermals, rather than a 100 L resevoir that is only there for thermal storage. You'd be much better off with a wax based PCM thermal storage in your resevoir anyways.
That's sounds pretty reasonable. If it was on tight enough or had other mounting issues, it would have higher idle temperatures. If you want lower temps you'll need a bigger cooler. That's a fine temperature for stock operations.
4K videos depend a lot on the Codec used and the CPU (if it must be software decoded.) Either way, as long as you have 4x connectors that can do 4K 60Hz, then there's no harm in just trying it.
A 9900K with a RX 570 is by no means a good value. Hyperthreading does not add much to a CPU anyways.
OP, the 9100F is faster than people give it credit. If you want higher performance in games, look squarely at a new GPU of RTX 2060 tier.
If stability of utmost importance, I'd reccomend 4x 16 GB, 3200 MHz matching memory, with as tight as timings as you can get. Consider ECC if your motherboard supports it.
Seeing every word capitalized hurts my brain.
Why would you risk damaging your new laptop and potentially voiding your warranty over negligable gain?
No laptop with a Ryzen 9 comes with an excessively sized cooler. Good laptop design involves closely matching the cooler with the CPU, to eliminate bulk while providing adequate performance.
If you want your chips to run cool, build a desktop with a cooler you specify.
Tell us more about the memory you're using. Part Number/Rev? Have you tested it in another machine since this problem began? Which slots did you test in?
Great, that answers one question. We know it's the BIOS and not anything to do with windows or the drive speeds.
Next, I'd suggest disconnecting all USB devices except a simple keyboard, and all drives except the boot drive.
Now that's... strange. Check if you have any other monitors plugged in (unplug if so, leave only a single monitor), try a different monitor on a different port of the GPU if possible.