The terminal has distrobox built-in by default. I haven’t tried distrobox myself (yet) so I don’t know the pros and cons, but it should at least provide some additional functionality. You can install “non-approved apps” via the command line as well.

Yeah, the custom app center is their way to have a certain “quality” control in the applications. For most users this will be fine, and for the other, mentioned distrobox might solve it, or I am sure there will be some more options in the future.