I did another round of tests, below 50% charge. At first, no luck with anything above 5 V 1.4 A, even not seeing a difference between the two USB ports. I was not able to eek out more than 5 V (or 5.2 V, if one believes the monitors) even from the Planet AC adapter.
Then, I turned the Astro off and then connected it to power in various combinations of cables, meters, power sources. There, I got more luck. The negotiation of higher-voltage modes is really finicky. Sometimes it works, sometimes not. Powering with the Planet AC adapter, and with my USB-A monitor, I've seen 9.9 V 1.5 A once. With the USB-C one, I'm more likely to get around 9.6 V 1.5 A (numbers might just be a bit off in the monitors).
After having established a connection that does 9 V charging, I turned the Astro on and it was able to resume that mode after booting up, with Wattz showing 14 W going into the battery. So this seems to be the highest mode possible, but getting that depends on a bit of luck, really proper connection with cables and all intermediate sockets … and maybe even a software state that remembers that the connection was tricky before and hence disables the higher charging modes to be on the safe side? Switching off the device resets things each time power is disconnected, I think.
On my DC-DC USB charging module, which supposedly can do all the QC modes and MTK PE, the best I get is 8 W charging at 5 V. The Astro is just not able to negotiate something higher. I don't have other devices that would test this … only the Samsung Galaxy S5 phone which happily draws around 9 W, too. I have yet to test what the Astro does on a proper USB-C AC adapter with PD.
The Wattz app is handy … when I'm pressed on time I'll check if the Astro is in 'safe mode' again and power cycle it to get back to the quick charge negotiation. I wonder if someone can confirm that there is such a thing as a soft disabling of quick charging above 5 V 1.5 A until next power-off. Maybe I just have a lottery with good/bad USB-C connector fit and wiring.