You don’t need Linux for this. You want Android. Buy a phone with a microSD slot, put a 512 GB/1 TB card in it, connect it to your computer over USB, choose File Transfer, and drag your MP3 folders directly onto it. Android treats them as normal files. Install Musicolet afterward. It’s specifically made for local music, works offline, supports SD cards and folder browsing, and handles MP3/FLAC/AAC/etc. A 2026 Moto G or Moto G Power supports up to a 1 TB microSD card. You can have literally hundreds of gigabytes of your own music on the phone without involving a cloud service at all.