Are you wanting to put songs in wav or flaac Format or something? I feel like a decently size used iPod wouldn’t be a whole lot more than $100 and 100 GB could hold 10,000 songs in high-quality 320 kB/s or so.
It sounds like you’re trying to build an iPod classic with Bluetooth
I DJ weddings for about 10 years and literally had my entire entire DJ library backed up on one