What's a sufficiently private/secure messaging platform? Signal comes to mind, but I'm open to alternatives.
Signal and Threema seem to be known for good UX and viability as everyday messengers.
There was this table: https://www.messenger-matrix.de/messenger-matrix-en.html
Get as many of your contacts onto Signal as you can. Disable the displaying of message text within device notifications from within the Signal app settings (for both Android and iOS).