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/
This is the solution I’m considering. Does it make sense if you have no actual contacts on Matrix? Do you think it would work on localhost instead of remote server? My use case is to get a single conversation view that includes Signal and Telegram contacts, but I don’t need it on multiples devices, one desktop box is fine.
I use it unfederated. Its on my local network server, running in docker with bridges to Signal, IRC, and Discord, no Matrix contacts. Works great for me.