There is a simple solution: buy UAPP.
On the other hand, i have H990N model and it always goes in hifi mode when i plug a headphone or other 3.5mm cable of a loudspeaker amplifier for example. it is actually called "hi-fi quad dac" mode. i cannot activate it if there is nothing plugged in but i can deactivate it while i use headphones. in this case i noticed the volume goes down, i didn't care to check if there is any quality degradation, it seemed the same just at lower volume.
About activation of hifi mode, i don't know if it's configurable by user. it might be in android config files, maybe someone would help you. there is only a menu activated on hifi mode allowing me to independently control the volume on left and right channels and that's it. No other trigger, setting involving impedance or anything else. At list not visually noticeable.
Now, UAPP related, i already mentioned there is a clear difference between sound quality using tidal own app and tidal via uapp, uapp is way better. i don't know if the same applies while listening your own files using stock lg player, it is possible stock lg app is good enough. But because lately i only use tidal via uapp and this app is recognized as one of, if not the, best, i stopped worrying about what lg does behind as uapp directly accesses internal hifi quad dac.
