I am happy to report some new options in this space.
AdaFruit now offers USB-C to barrel jack cables. They each support a max 5A draw, are one meter long, and cost eight dollars apiece.
The interesting thing is they’re available in different voltages:
I have now acquired all four variants and can confirm the Reform will charge using any of them. I tested all of them with a PinePower Desktop supply, which doesn’t supply more than three amps for these voltages, but the Reform happily charged from all versions of this cable.
Enjoy!