I'll give you the strategy that works for me.

Do NOT talk about privacy, E2EE, or any of that. To regular people, "privacy" has negative value, so even if they wanted something, your talk of private communication and not being scanned by data brokers and governments will make them want it less.

So what to do then?

I present Signal as a novelty and focus on one single feature: "hey, video calls work much better than WhatsApp! Try it out!"

Bam, instant download.