Are there high quality MP3 players with an open ecosystem?

While "portable MP3 player" implies "not a cell phone", have you considered use of an old cell phone and an open source music player (i.e. one example here: https://f-droid.org/en/packages/ch.blinkenlights.android.van...).

The "phone" does not even need to work as a "phone" anymore, it just needs to work enough to open the player app and play music.

And with the example player above, it "does not require a user hostile music ecosystem". It simply reads the phone's filesystem and catalogs all mp3 files it finds, and then lets you play those files.

It (using either an old cell phone, or your current one if you have one) will allow you to "just to be able to load and play mp3".

Find a Sansa Clip, Clip+ or similar on ebay and install rockbox on it. Once setup you can just plug it into your computer and drag & drop mp3s like any other USB thumb drive. They're tiny and it's great having physical buttons; you can pause, skip, etc without looking at the device.

There are a bunch more supported players but I've only used Sansa. See: https://www.rockbox.org/wiki/TargetStatus

You can buy a small and cheap smartphone based on the MTK processor. Another option is to buy a Chinese board with a 24-bit player. I will explain the difference. When you play music on a 16-bit player, including the sound card of the computer, you hear a stepped amplitude of the sound, which generates harmonics. Distortions are introduced by integer or float arithmetic, so here you need to use hardware volume control, based on field-effect transistors. Many years ago, in 2009, I made a program for Windows XP and Direct Sound based on the mpg123 library with a patch for automatic volume control at the decoder level. It was really high-quality 32-bit sound, which was used on a local music radio station. I ported this program to an Android tablet, but did not finish it.

I've used a number of Philips SA2208 8Gb players, with headphone jack, for this. The simple display seems to get patchy after a while, but the music playing function seems to work quite well for my purposes. I'm not aware of any third-party firmware support though.

I've been looking into the same question, but so far have some up empty. I'll be keeping an eye on the answers here for anything I may have missed.

I currently suspect that there are no suitable devices on the market and will need to build my own.

> device from a known manufacturer.

See, this is the problem here. You can buy a Walkman technically if you want to cough up $75 for a glorified feature phone: https://electronics.sony.com/audio/walkman-digital-recorders...

If you're dead-set on finding something that checks all your boxes, mainstream manufacturers won't cover it. This is one of those things where I'd search it on Amazon and sort by highest public reviews.