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I would like an audio device which can play mp3, podcasts, internet radio. Bonus points if it supports some kind of cartridge system, size between credit card and audio cassette. Thank you for your attention to this matter.
You're about to respond: "But many of these use Android, and general purpose computers are too distracting for me." In that case, you'll need to forego live internet audio streams and buy a closed option with a radio receiver.
Get a second-hand Apple iPod Touch, remove all apps you don't need.
For just mp3 and podcasts: get an iPod Classic (or Video) and install Rockbox.
Rockbox is amazing.
That internet radio is a whole magnitude of complexity, especially with the need for wifi (cellular?) if it needs to be portable. But there are options like specially modified android devices.
I have the Shangling M0 with a 512GB card. I don’t even bother with converting my flac files. A nice other device is my kobo. It holds my entire fiction library with space to spare.