PseudoRandomGermanPerson

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

So, the last time I actually did it was quite a while ago while reparing my old google pixel 3a. I was testing the new screen before putting in a new battery and closing it up. But as I said, I am reasonably sure any android phone will allow for this, specifically for repairs/disaster recovery etc scenario.

Also, since I don't know your actual requirements, you might wanna consider powering it through the battery connector. you can find premade adapters for some phones on aliexpress, or can rip out the BMS and solder a power supply to it. But thats more involved, and USB Power will absolutely suffice

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

Oh no, project sekai has broken containment and jumpscared me on my fediverse timeline

( I think this meme might be a useful template for this community )

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