Third time’s the charm. I’ve ordered the Anker USB-C to 3.5mm adapter. It’s small, sturdily built (braided cord and all-metal build) and not too expensive at €16. The cord is much thicker in fact than those of the Sharkoon and Apple adapters.
And drumroll 
: no noise! In fact, the sound is very good. Oh, and the pause/play/skip button works. I have several peripherals from Anker and have never been disappointed in them 
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My aforementioned 54 ohms Sennheiser HD380 Pro are also very easily powered by this adapter. Volume is at about 50%.
Weirdly, Anker doesn’t advertise that they make these adapters. You can’t find as much as a mention of them on the Anker website. In fact, only the US & German Amazon stores seem to sell them 
. The box it came in was utterly unremarkable, white with just the Anker logo and their slogan. They were apparently launched early last year, and look remarkably like the Ugreen adapter.
Oh well, I’m happy. I can wholeheartedly recommend these adapters to anyone looking to buy one.