I'm planning a very similar build (3990x + 3x 2080 Ti), registered here just to comment.
1. I would be hesitant to put the Noctua NH U12S in a 4u case. Contact between the heatsink and the roof of the case means that any pressure above the case will push on the motherboard. Will you have full control over the rack? Also, in tests by phoronix.com, the U9 actually slightly outperformed the U12 - the U9 isn't as tall but it's deeper and weights just as much. The U9 with higher-RPM fans seems like the best choice for 4u.
2. Another option is to move up to 5u (and then you can just place a desktop case like a FD Meshify C sideways on the rack). I'll be colocating, so that would be a significant bump in price. I would only do this as a last resort.
3. I also plan to look into underclocking the CPU. AMD is setting the out-of-the box PPT to optimize much more for peak performance than for efficiency - these chips are effectively overclocked out of the box, as HEDT chips tend to be. If you're running 24/7 then lower clock speeds will almost always give you a better lifetime performance per dollar. (The more you use the CPU, the more your lifetime cost is dominated by power consumption and the more sense it makes to underclock.)
Some links:
https://shipilev.net/labs/threadripper-efficiency/
https://wccftech.com/amd-ryzen-threadripper-3970x-32-core-efficient-monster-cpu-eco-mode-tests/
The exact optimal PPT will be workload-dependent (and chip-dependent, etc), you'd want to run your own tests. If you go down this road then it also makes sense to look into underclocking the RAM.
