@maxmp well, I don't know how was it a year ago, but, at least in my phone (redmi note 9s), UAPP works fine, and I have a custom rom (it works fine with stock rom too); I mean, the playing is smooth, no disconnections or anything. But the app interface is not the best and is a bit slow, that's true.
So, is not possible an option to ask the user to allow direct connection to the external DAC? Like this app does. Sure, it would need some kind of driver, but wouldn't be possible some kind of general driver for external DACs, I remember that I read that new USB C ports have advenanges over old micro USB connectors, so, it should be easier.
The DAC is HIBY FC3.