You're probably right prime95 is causing it to thermal throttle, but honestly I don't mind cause it maintains its clocks at 4.7 ghz no problem during gaming which is mainly what it's for. I have a 9700k on order and may move the 9900k to my midtower build, maybe that'll run slightly cooler.
Not sure if the black ridge will fit, what should I measure? Even the cryorig C7 CU was quite a tight squeeze, barely fit and presses against my ram. Definitely an interesting proposition with having the 120 mm fan with the side panel off, maybe something I'll consider doing once I get the black ridge. I did notice it was running cooler with the side panel off.
Not planning on delidding it as I'm quite happy with the performance currently. If I had gotten the 8700k I would have delidded but with the soldered TIM I don't think its worth it and it's a higher risk to delid. I'll see how the AIO 92mm does once I get my custom cables.
That's a smart move, the 9900k is a really hot CPU and if you can move it to a bigger case with better cooling do it, you'll get a lot more from that CPU like that. We were all expecting Intel to solder the CPUs to be better than liquid metal but I guess not... But I would suggest to hold your purchase on the 9700k and see how the Black Ridge performs (with the setup I suggested and if it fits with the z390-I) to save money.
The Black Ridge is 140mm x 120mm x 47mm, and from I can see they suggest to mount the cooler with the heat pipes (the 180-degree turn) facing up, over the VRM, so there should be no clearance problems here.
Width is the problem here, it's 120mm and looking at the picture and if my math is correct, the area over the ram should be around 20mm which is also not a problem. The area on the opposite side should be 8mm from the end of the fan to the motherboard I/O cover. The fan seems to be centered above the heat plate of the cooler, so, center a 92mm fan and measure the space between the fan and the I/O cover. Take this information with a grain of salt since I don't actually have the official specs and I don't have the cooler to measure it, but measure it anyway if you can.
And take a look at these:
And yeah, I'm actually considering getting an 8600k and delid it instead of the 9600k, the only problem is that I wanted to keep the warranty and have the Meltdown and Spectre speed down fixes (If they even exist... They're just a rumor and I can't find anyone who can confirm it).