I probably will try that 2.5.1 build. Smaller rockbox seems nice for hacking if nothing else, and I'm not sure what I would gain from using 3.13 or for that matter what I would be missing compared to later releases; maybe someone can explain.
There are detailed release notes:
https://www.rockbox.org/wiki/ReleaseNotesI understand it's an ancient device but I couldn't find anything else supported by Rockbox with an easily replaceable battery. I was sick of prying and mangling iPod cases in order to replace the battery only to have it fail in a year even from supposedly reputable vendors.
Just off the top of my head:
* iBasso DX50 and DX90 use an off-the-shelf removeable battery. (Samsung EB-L1G6LLU)
* xDuoo X3's battery is easily replaceable (remove two screws to open the back)
* ipod mini batteries are pretty easy to replace (no clips) and replacements are plentiful
* erosq family of devices is easy if you have a heat gun
* Hifiman HM-801 also has a removeable battery, but I don't know how easy it is to find replacements.
(EDIT: Add HM801)