I usually have two problems using darktable (due to my lack of knowledge and practice): dealing with highlights and denoising.
It looks like I can’t use Denoise Profiled to get natural-looking results while keeping a good amount of detail. I usually just want to reduce the noise to make it more pleasant (and not suppress it, giving those plastic results), but I find it difficult to do it in DT. I always have to play a lot with the denoise profiled’s parameters (following rawfiner’s videos about it), but the module takes a loooong time to process on my machine, I get tired/out of time, and I usually end up going back to RawTherapee when I want to edit a high ISO picture (if I only want to do basic adjustments to an image and denoise it, I use Capture One Express, I really like its denoising tool).
The same apply for highlights: I always have trouble with them in DT, but they are rarely an issue with RT.
So, since I prefer using DT for almost everything, I would really like to learn better how to deal with these issues, and would be nice to see your edits. The photo I’m posting here is nothing more than a snapshot, but I thought it would be useful as a test/learning tool. I’m posting two JPEGs: one is my edit on DT (which I don’t like at all), and the other is from capture one express, just to compare (I know it’s not fair to compare two totally different softwares and that results can’t be copied, but it’s just to give an idea of how it deals with details; pay special attention to the details on the top of the building).
About the de-noising: I just migrated from an old, slow machine to a newer, more powerful one. I do know your frustration, especially with this kinda shot. Most of the time you can get away with applying the de-noising at the very end on a less powerful machine, but in this case you do need at least a decently de-noised image before you can do some work.
On a side note, you could have probably made your editing live a lot easier in this case by changing the camera settings a bit.
Since there is so little light and nothing relevant in the foreground, the reduction in noise is very likely worth the optical quality loss of stepping down. So rather than F8, i think shooting wide open would have made sense.
There are clipped highlights on the building, but you are at iso 12800. If you would have shot this at say iso 1600 instead and increased the exposure in post you would have just as much signal but no problems with the signal clipping.
Back to topic, here is my shot at getting rid of the noise. I ‘fixed’ the out of gamut colors by setting the input profile to linear rec709. Nope that’s not correct. The colors are all out of whack but you asked about denoising and recovering details anyways.