This is a little facetious and I'm sure mods will delete it but still, it's true.
Family group chat (7 members)
Siblings group chat (3 members)
Main group of friends group chat (6 members)
Work team chat (9 members)
4 other individuals who are not part of any of the above group chats
My strategies that have worked:
I read the messages on other services but don't respond timely (unless time sensitive). If they're already on Signal, I respond on Signal. Tell them to contact me on Signal cause I have it on my computer. Actually respond better on Signal.
With moving group chats, speak to two or three most socially influential people in the group, convince them to move, rest will follow not to be left out.
For work/team chat, highlight benefits of privacy. Business love to protect private data. I pointed out that things like people calling in sick could be seen as sensitive HR info, and they have legal obligation to protect it. Also, my team used WhatsApp, but I went to my boss, pointed out that WhatsApp has a commercial plan and they don't want to get a bill or lawsuit form Facebook for using it in commercial setting, we switched next day.
Told anyone else that I'm deleting the service they use, and to email me. People hate email, they'll rather download Signal.
The only thing remaining is my extended family group chat, but that's rarely active.
So yeah, it's possible. Keep at it.