I have a few friends and family on Signal. Pretty much everyone uses other services too (including myself) but Signal is installed on a few
I talk to some friends / family on Signal exclusively. They either already cared about it, or they’re close enough that they trust my recommendation / still don’t care but want to put in the effort
I talk to some friends / family on Signal only when we want to have a more private conversation
Some people won’t ever be convinced, I talk to them on other services or in person when privacy is important. I set up boundaries to protect myself, and people usually respect that.
This way I protect myself while respecting other people’s choices.
For you I’d recommend focussing on the second option. When you need to talk privately, ask the person to install it and give a short explanation for why it’s better. If they do, then great it’s installed and it’ll get used once in a while. Maybe they will see other people and build up the network. If they don’t install it, then suggest an alternative like talking in person