Darktable processing is so much easier to get quick, satisfying results with nowadays. Big shout-out to @anon41087856 for filmic, especially. The performance of denoise in auto-mode is now so consistently good I very rarely fiddle there. I suppose I could have masked and desaturated or biased-down the bright background, but that is part of the framing of the subject, showing how well it camouflages itself.
The (totally uncropped) 3:2 tends to distract my eye due to the brightness of the background, so I tried the square crop, but that is unsatisfying also.
I crop the photo the way I want to see it, and then use GIMP to first change print size and then change canvas size to the size I want for printing. When you set canvas size, you can choose a color for the border. I am using Windows, so I don’t have the dt print module, or if I understand right, on Linux you can do the same thing within dt. Then again, maybe not.