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SimpleX has the same limitation. I hate how all those apps with modern networking or modern crypto can't get those important usability things right.

Briar runs a Tor Onion Service on your device. I'm unsure how well Tor supports having multiple devices acting as the server. One option could be to have a main program that's always on, to which your messages are received, and other clients can then use the Onion Service to fetch a copy of the conversation. It would add another link in the chain and increase latency so I'm not surprised if that's not being done.

Briar offers a companion ”mailbox” app for the exact reason you describe (i assume)

where did you read that it hosts an onion service? i guessed that's how it worked but couldn't find anything in their docs that said so.

Identity is tied to the physical Bluetooth MAC if I recall correctly.

can't that be spoofed/overwritten?

The BT MAC is shared with peers so they can find you (as well as a list of your IP addresses) but I don't think identity is tied to it. Identity seems entirely tied to a public certificate. But I could be wrong.