My approach: import unsharpened image from Darktable, scale down with NoHalo (added in GIMP 2.10, along with LoHalo), sharpen final image with unsharp mask. When downscaling a clean image, try 1-pixel radius, strength 0.35, threshold 0.05 as a starting point.

"None" interpolation is occasionally useful as well, in image with really striking edges.

Agreed that the older downscaling algorithms aren't great. My impression is that they do a nice mathematical job of downscaling, but not a great perceptual one.