Recently discovered this. Molly supports link with existing device just like on signal desktop. It even has benefit of getting entire chat history unlike signal desktop. Just restore the signal backup file during setup and then click link with existing device. Then scan with you primary phone. Beauty of open source. Molly: https://molly.im/
F-Droid doesn't want to host Molly because Signal doesn't want any forks on F-Droid. Seriously, that's the whole reason. Molly devs would be fine with it.
Molly is actually reproducible and has a fully FOSS version, so it is trustworthy.
Is that because Signal has trackers in it and F-Droid would remove them?
Wait, Signal dev forced f-droid admins not to upload Molly to its official repo? How?
No. The F-Droid team just doesn't want beef with the Signal guys.