that doesn't hide metadata, the postal service can track it and you can trace back both the handwriting and reverse engineer the code. Someone can intercept the mail from many stations on the way, including your own post box.

I'd say that many messaging services offer more protection than that method.

You could, however, use PGP with elliptic curve cryptography, and send that via packet radio or something similar on a frequency only you know. To an uninvited person this only looks like garbage data or noise.

Reddit provides a neoliberal space, Raddle/Gab/chans (4chan et al) provide a far right safe space, and Lemmy provides a safe space as well.

I think you misspelt "echo chamber" as "safe space".

Way too much choice. Do you think uninformed users make the right choices? Do you think all people you interact with are capable of gathering information or even willing to think about what app to install, what server to choose etc.?

If you try Signal.org or WhatsApp.com or Telegram.org, you see that the streamlined walled garden is way easier for people to sign up to. Just count the clicks from Matrix.org until you can chat with someone compared to one of the aforementioned apps.

Why not just contribute to Matrix, which is already well established and tries to 'discordify' their ecosystem, too?