Loudness control is kind of built-in already, no?
In the WiiM Home app go to Device -> Settings (gear icon) -> EQ. Create a loudness correction curve of your liking, hit the [Save] button, save to "New Custom EQ" and provide a name like "Loudness" or "Contour" or whatever this has been called in the olden days. Enable when listening at low levels, disable for live-like levels. Job done, trouble shot.
It's "fixed", not "dynamic" in the sense of it doesn't automatically adapt to the current volume setting, but that's OK. Equal volume settings don't translate to equal sound pressure levels anyway, if the speaker's sensitivity isn't taken into consideration. Speaker sensitivity can easily vary by 10 dB SPL (roughly equivalent of twice the perceived loudness) or more. For this very reason dynamic loudness control never works perfectly, maybe except for all-in-one active speaker systems.
You can still save two or more different loudness control EQ settings for different average levels and
you decide how much (or not) certain frequencies are to be boosted, so I would still call it "variable".
In any case, I would rather call this a feature request (which I personally wouldn't vote for) than troubleshooting.