Thanks for the infos.

For both lights (Front: Union, Rear: B&M), there were no instructions on the packages, hence my questions about wiring, and the meaning of the two OUT plugs on the front light.

Here's a smartphone audio recording of the Axa HR Traction with the bike on a stand, with the dyno rubbing against the rim of the rear wheel (disk brake, FWIW):
https://vocaroo.com/i/s1KRiFGAerQV

Neither particularly silent nor drag-free, but definitely better than bothering with USB light.

I'll probably build a wheel later with a Shimano or SP hub dyno, and reuse the lights I bought. I learned useful tips in this thread, thank you.

--
Edit: Out of curiosity, I reversed the two wires on the tail light, and it lights up. So, unlike the front light that has an ON/AUTO/OFF switch, it makes no difference how the tail light is wired (OUT thunder and OUT ground).