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FWIW, Whisperfish, the unofficial Signal client for Sailfish OS works really well. An if I wanted, I could use the WhatsApp Android app on Sailfish via its App Support emulation layer.

https://openrepos.net/content/rubdos/whisperfish

With some caveats, I also successfully used whatsapp over matrix although that required to have whatsapp running on a separate device.

Punkt's MP02 supports Signal.

https://www.punkt.ch/en/products/mp02-4g-mobile-phone/

The 3G and 4G versions of that phone are Android-based, for context.

That's disappointing to learn.

you could set an away message to point at other means of messaging(?)