I have a few friends and family on Signal. Pretty much everyone is also on other services (including myself) but the option is available

• I talk to some friends / family on Signal exclusively. They either already cared about it, or they’re close enough that they trusted the advice / 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 just won’t ever be convinced, I talk to them on other services or in person. I set up boundaries to protect myself as needed

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