The Signal project doesn't want non-official clients to connect to their network. I doubt you'll get them to release an official client for your bespoke dumbphone OS, so I'm afraid you'll have to accept unstable hackery as a way to run signal.
You may be able to create your own dumbphone by flashing something like PostmarketOS to a compatible second hand phone combined with a custom Signal client. Just apk remove the browser and any other utilities you don't need and even the librem phone might be fast enough for your use case.
You may be able to create your own dumbphone by flashing something like PostmarketOS to a compatible second hand phone combined with a custom Signal client. Just apk remove the browser and any other utilities you don't need and even the librem phone might be fast enough for your use case.