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/
Yea, the mobile device requirement is so pointless and annoying! I have to use Signal with only a couple of people and had to use signal-cli, which is pretty annoying because it doesn’t display history. Now I have it on Waydroid, but you should NOT have to do that.