If you can find one and have the cash and skills to do the mod, find an older iPod, such as an iPod Classic or iPod Video, and then SD-card-mod it and flash it with Rockbox.
Those older iPods still have a really good audio section even by modern standards and operate completely offline; Rockbox just 'unlocks' them, as it were.
Also, USB-C-modding them would be recommended as well in order to ditch the proprietary cable.