I don't understand, why don't you force them to use Whatsapp (or Signal, or whatever) to contact you? Get an app that rejects by default SMSes coming from certain numbers. They want to text you at all? They need to use Whatsapp, otherwise they can go fuck themselves. (It worked for me when a friend wanted to force me to contact him on Telegram rather than Whatsapp- I resisted for weeks but at the end I gave in).
Once you automatically reject SMSes from those contacts, such that you don’t even know they're trying to contact you, the ball is entirely in their park to take action.
Once you automatically reject SMSes from those contacts, such that you don’t even know they're trying to contact you, the ball is entirely in their park to take action.