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

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