It depends on what exactly you’re looking for in a messenger. If you are able to get people onto a specific platform, go with something like Signal, that’s your best case scenario.
If you’re unable to do that though, and need SMS/MMS, you have options. QUIK is an app I highly recommend for SMS and MMS. The big downside, however, is that RCS is seemingly exclusive to Google Messenger on Android. If you want to use RCS, you’re kind of stuck unfortunately.
To minimize needing to use it, you could buy an old iPhone, jailbreak it, and try and set up Beeper Mini on an android device. That’s what I do at least, which helps quite a bit. It’s finicky and just not perfect, but it’s better than just using SMS/MMS.
Hopefully this comment covers your use case and you’re able to get some useful info out of it.
QUIK is a more up-to-date version of QKSMS. Not the most important thing ever, since SMS/MMS are inherently insecure, but it’s just good to always make sure you have up-to-date versions of things.