"secure" is relative. They may not be e2e encrypted, but they are still encrypted via TLS, like any HTTPS traffic. It's the same encryption used for online banking. If you care about your instance admin being able to read your messages, you should use Signal or a Matrix client though.
But remember that only a few years ago, almost nobody used e2e encryption, and it wasn't much of an issue.
I've been using Manjaro with KDE for a few years now. It works smoothly, I never ran into any issues with it.
The
pacmanpackage manager is pretty nice, too, I found it faster and easier to use thanapt-get, and the provided packages are always kept up-to-date. Updating the system (even installing a newer Linux kernel) is very simple and works reliably. So you always have the latest version of your apps, the kernel, and the DE.In the rare occasion that a program is not available in the official repositories or the community-maintained AUR, you can also install
snaporflatpakpackages.And since Manjaro is derived from Arch, you can use the Arch Wiki, which is very useful when you want to set up a database, use the android debug bridge, install another package manager, or do anything else less than trivial.