Mastodawn

@cocaine_owlbear
One problem of asking such a simple question in this place is, you'll get very complicated solutions and people would ask you to self host their favorite solution, than suggesting something practical.
For your usecase Signal seem to be most appropriate solution, as it is more private (no meta data collected, does not expose your phone number) hence more secure. You could find a detailed user guide at #EFF site here to help you take advantage of all signal features to make it's privacy more robust.
ssd.eff.org/module/how-to-use-signal
And yes, signal runs on all the platforms you mentioned (however being a mobile first app, you'll need an android phone or an iphone as your primary device and need to use a phone number to register).

How to: Use Signal

Download location: Google Play Store, Apple App Store System requirements: Android 5 or later, iOS 13 or later Version used in this guide: Android: 7.38.6 iPhone: 7.5.1 License: GPLv3 Level: Beginner Time required: 15-20 minutes Other reading: https://signal.org/ https://support.signal.org/ https://signal.org/blog/ Table of Contents Download and Install Signal Register and Verify...