How would this help if OS manufacturer or even baseband operator could read anything in your phone's memory remotely, including encryption keys?

I agree with /u/na85. If you're worried about law enforcement or corporations sniffing keys out of your devices RAM, you have other things to worry about besides "what app should I use for communication"?

If you're seriously worried about communicating without trusting your phone, then you either need to find communication devices you do trust, or else look into verifiable TRNGs [0], dumb serial printers that can be directly attached to RNGs [1], and either manual encryption/decryption algorithms [2] or one-time pads.

[0]: https://archive.is/CGQkR

[1]: http://www.loper-os.org/?p=2146

[2]: https://eprint.iacr.org/2017/339.pdf

I simply want real privacy, not an illusion of one. It's pointless to use any half-baked privacy approach when your device is compromised by design; who knows if the current mainstream becomes dissent in 20 years and people will be hanged because of their old communication logs? Thanks but no thanks.

That's moving the goalposts.

Pick a threat model and stick to it.