I tried with Telegram after failing with Signal and that worked for many. For whatever reason (I assume the user experience is nice and more compatible with Whatsapp etc), non-techie people do prefer Telegram to Signal in my experience.
I've gotten my girlfriend (now wife) and parents to switch. My siblings use it sporadically, but I think mostly to talk to me.
Additionally, taking the initiative for fun activities (or always being eager to join), which -besides being fun- gives people without Signal FOMO, haha.
Edit: Well, "switching" is a big word. They've got it installed, use it to contact me, and some have started using it as the primary means of communication with others who also have it. Most of them will still use WhatsApp even for contacts who are also on Signal though.