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/
I got a chunk of friends & family to Signal a couple years back. Now I have some regrets due to the architectural decisions of the Signal ecosystem (and that battery drain + Electron app being huge). I wish I had had resources for an XMPP server as even a lazy person could run Snikket, but now they don’t want to remigrate after on a few years.