Another update:
I figured out that instead of merging the L channel of nlmeans and the AB channel of the directional pyramid denoising, I can run directional pyramid denoising on the nlmeans output.
It’s pretty good, but it has some weird artifacts that remind me of a color-inverted blue-ringed octopus…
The better parts actually look quite fine, though, and it is in my opinion quite outstanding on images that are not as noisy (this is ISO 25600 on an a7r4).