There are efforts to get Signal working on the PinePhone and Librem phone through Anbox. Signal also does not require Google Play Services, it can run with completely libre software all the way down. So, Signal is becoming a viable choice on a device that meets Free Software standards and is not a “spyphone”.
I think (but haven't tried myself) that you can nowadays even confirm your number on desktop, i.e. receive a text message on a dumbphone if need be, and then enter the confirmation code on your desktop.
