The F/OSS part is the most important. Rockbox allowed an incredible degree of customization. For something like a music/audiobook player personal tastes in interaction with the a small device mean that configurability is key. That, I suspect is why there are only larger devices with loads more buttons and touchscreens available.
The ability to set timeouts and restart from last position on power-on, led light-level, sort-order etc made devices supported by them something that I could use. I have tried various Alibaba sourced devices that were in the US$20-50 range and none of them had the right form-factor and customizability.
I found an Android app called MortPlayer which is very close to Rockbox in terms of functions and configurability. It hasn't been updated in a years, but still works well.