I'm also really into single-purpose electronics. My whole library is FLAC, so these players should be right up my alley. Except they tend to be heavy and expensive, and when it comes to UI, either they're using Android poorly, with incredibly bad battery life (HiBy R5/R6, Fiio M9, A&K anything), or they're using a custom UI with pre-iPod UX sensibilities. And even when they use Android, they try to graft their own UI onto it, so that in the end you have the worst of both worlds -- just watch what happens to the volume settings on any Android-based HiBy DAP after connecting with Bluetooth.
The communities that buy this stuff are much more into the technical and theoretical sound quality than how it actually comes together as a product. The marketing reflects that; the product pages are all litanies of incremental DAC processor upgrades, circuit diagrams, and cryptic audio codecs. And because this is what folks care about, it's hard to find reviewers who even mention into UX.
If you have a rec I'm all ears, because I'm on the verge of resuscitating my original Pixel just for this.