I used to use Rockbox on an old Sandisk Clip - it was great and did exactly what I needed. Then it died and I discovered that Rockbox doesn't work on the newer Sandisk devices.
You'd have thought that the few basic features that I needed would have been implemented in their interface by now, but you still can't tell it to play all your albums in a sensible track order. Selecting Play All in the albums menu doesn't play all your albums in album order, instead it plays track 1 off all the albums, followed by track 2 off all the albums, then track 3...
In the end I had to fall back to writing a simple script to create a playlist of all the tracks in album order - it's not a great solution but it's workable.
Personally, I rebuilt and refurbished an old iPod (new battery, upgrade HDD to flash) it is a bit like a classic car.
I dont require audiophile DAC, I just need decent battery, as few features as possible, and massive storage. It is a plus also if I can just address it as a drive rather than needing itunes as it is a little flaky on Linux.
One thing that piqued my interest is the new raspi pico I think it is called. Not sure if anybody has looked into it yet to any extent, but at least the price and form factor are right