What about Rockbox? This has been kicking around for years. Read https://www.makeuseof.com/tag/ditch-mp3-players-firmware-favour-rockbox/
It's dated but the players are still out there and firmware appears to still be out there.
More devices at https://www.rockbox.org/
I was wondering if anyone has tried this. I've gotten increasingly frustrated over the years with the Sansa hardware, but still find they have the best OS, especially when it comes to podcasts.