For me WhatsApp seems like a decent compromise although I'm not aware how many people in the US actually use it or have it installed. The iMessages doesn't seem like a valid concern in Europe. Here almost all people I know have WhatsApp (and virtually all have Facebook / Messenger).
Myself, I prefer to use Signal (Molly actually) and I initially offer everyone to use it but if they decline or are unwilling to try it, I settle for WhatsApp and move on. If one day they inquire about Signal or we happen to talk about privacy, I explain my reasoning and the advantages. Very few of my friends either installed Signal when they met me or after a while but most haven't and probably never will and that's fine.
I believe positive motivation (what you will gain by using Signal) rather than the negative one (I won't contact you if you don't) is the way to go to convince the normies, even if most of them will never change their mind. I'd rather not discourage the few ones that care enough to try.