My janky(-ish) but fun plan is to use laptop motherboards, either from broken laptops or AliExpress. Wire them up with an ATX powersupply and that should be a cheap and powerful cluster. If you want something less hacky the same idea but with used mini-PCs from offices.

Well, if you use DDNS I assume you have some kind of server behind that. I just self host a godns container. No need for any service except an DNS API. I use cloudflare. But my IP only changes rarely, so I can’t tell you how fast that setup propegates

What is your goal? technically any Android phone can do it, just remove the internal battery. Some might complain, probably most will complain, but they will start and work

Incoming infodump. I agree with another commenter that Auracast ist exactly what you want, problem: barely available.

Bluetooth is split into 2: Bluetooth Classic Bluetooth Low Energy

Auracast is a very new standard using BLE Audio. I’ve seen some aliexpress sellers selling Auracast hardware for silent discos. But from well known brands I haven’t seen a thing

Most BT headphones you buy use Classic, and cannot achieve multiple connections. So no built in solution.

Obviously there are proprietary Technologies that had custom transmitters/recievers for this exact usecase.

TLDR: If your Phone can do BLE Audio you could gamble on some Auracast hardware from aliexpress. Else, look for premade solutions not using Bluetooth