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