>how to secure SMS/MMS traffic
gpg mms is probably the best you can do.
using sms relies on the cellular phone system which relies on multiplexing which relies on sim cards which relies on a customer id number and carrier id which is attached to you and can be used to locate your position using base stations and satellites in like 5 different ways.
you can register any phone number on signal and use it from a web browser so it's a little bit safer metadata wise. Using the google push notification framework makes sense as it's lighter on battery than sockets and there's not really other infrastructure out there for droid.
Plus it has great PR unlike secure.im, Your friends may have heard of it.
If the object of the game is to chat securely with as many lay people as possible then pick signal every time.
Always validate fingerprints though and be wary of whatsapp and telegram because the implementation is bad. i.e. https://www.schneier.com/blog/archives/2017/01/whatsapp_securi.html