I was able to get grandma on Signal precisely because it was the little secret between us. She knew that a signal coming in was from me, and only me. I was using it to talk to her. And she thought it was cool that we talked via signal to her before any other family member caught on the signal craze.
I was able to get my SO to use signal on the same principle. Pick the people in your life that care about you - and care back. You are available via signal, while you ingnore whatsapp et al. You answer right away on signal whereas on other stuff, you wait.
Its funny how far along you can take a feature-lacking app provided its simple for others to use, and there is an emotional attachment in its use.