I export to jpeg-xl because, apart from supporting HDR and wide gamuts, produces smaller files than, for example jpeg, using similar quality settings.
I now choose 16 bit because I have just learnt that 32 floating point is overkill for my need.
Bear in mind that, in order to be seen exactly as you see it, the end user must use the same display and the picture must not be automatically converted to a SDR jpg, for example, when shared via Whatsapp or similar.
So when I occasionally share an HDR picture which looks great on a P3, I have no idea on how it looks like on an SDR phone.
Regarding the brightness of the highlights.
Since darktable’s preview is not close enough to how the image look like once exported (not even on the same screen), I rely on the histogram, as I said. I put the wide gamut tool / right-click: histogram profile to HLG P3 which in my case is the same as the export profile (HLG P3 RGB). Then I take care that my histogram never crosses the right border. In fact I try to keep it at a short distance from it.
Regarding the colors.
Again, the issue is that, even with my darktable built against the patched gtk+3, the preview in Darkroom is not faithful to the exported image as it appears in the viewer. So It’s really a matter of trial and error. I guess that you could try to desaturate a little bit the highlights, or reduce the chromaticity, for example with Color Balance RGB and learn from experience.