I’m giving this little “cheap and cheerful” box a chance: “Moukey Ultra Low-Noise 4-Channel Line Mixer” (Amazon) (Aliexpress). For power it asks for 5V/1A over micro USB.
For a quick and dirty 4-channel stereo-capable “mixer” it’s …fine. Utilitarian, that’s it. Haven’t pushed it to see where it really falls apart, but all I wanted was an option smaller than a K-Mix, 3 line-level instruments in stereo, battery-powered as an option (not passive), and in stock (at the time). If I’ll need proper balanced inputs, metering, etc, then I’ll need the K-Mix or larger anyways.