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’m in the same boat. To be frank, if I didn’t have to use WhatsApp to talk to those in my family that remain in my motherland, I would be more than happy to get a simple dunbphone.