Upgrade. Use Element
Discussion
A half a dozen extra steps to set up accounts for the grandmas compared to signal with less reliable message notifications which will cause them to fall back to other messengers.
The grandma went from letters, to landline, to email, to messaging apps. I'm sure she can be taught a couple more steps. Anyways, if not now, evolution will eventually take you (and her, if she is still alive when the network effect picks up) there
And it's awful when you use (god forbid) a macbook and a pixel. Husband set it up for me twice, I broke it down twice - security warnings block incoming messages, it's not really usable. At least year ago it wasn't.