The most secure way of messaging is becoming thoroughly versed in infosec and only conversing with other deeply security minded individuals using E2E encrypted apps.

In lieu of that, use signal. Matrix if you want some more features. But never for a second assume that using a “secure” platform or service makes you secure. Security and privacy are a never ending dance. The most important part of which is to keep moving constantly. Understanding new threats, keeping a close eye on updates for your client of choice, and maintaining a constant background suspicion will ultimately have a much greater impact than which platform you use.