You’re not going to convince anyone to suffer inconvenience for something that has no tangible benefit in their eyes. The best you can do is give people the option to contact you on Signal and explain (briefly) why you prefer it. After enough experience, you realize there is no argument you can make that will convince people to care about privacy. The people who join you on Signal either already care about privacy (but maybe didn’t realize it) or value your comfort over theirs.
Personally, I would rather send unencrypted SMS instead of using a Meta-owned service. I don’t want to be part of the network effect keeping people on Facebook. Everyone with a SIM card in their phone already has access to SMS, but few use it if they can help it, so I don’t think I’m contributing to a network effect by doing this. The only MMS client I use is Signal, so anyone can contact me over there if they want more functionality. That’s the only tactic I use, and so far, it has been unsuccessful.