I can recommend the FiiO M3K, especially loaded with https://www.rockbox.org/ . There is now a native Rockbox build (with some caveats), I use the older one from https://forums.rockbox.org/index.php/topic,52952.0.html - works very nice.
I use the older one which uses the kernel from the stock firmware. To my understanding done by the guys who worked also on the original firmware. I have seen just a single issue, it's minor: fast forward/rewind does not work properly. It does forward/rewind, but slower than expected.
The new native port lists quite some downsides/TODO's, so I did not yet switch over.