In order the user frontend looks the more simple as possible, maybe you can hide by default the settings "Notification level", "Hide topic messages" and "Enable infinite scrolling".

A button with by example an gear icon allow to show these settings by expanding the left panel. It saves place at the screen.

The more important for the user is to have the finder tool, the name, the icon and the short description of the topic and some statistics data.

Else, another solution can be to hide only the "Notification level" and the "Enable infinite scrolling" because the "Hide topic messages" can be usefull in direct access.

The "Notifical level" is very important since that will be used to subscribe to any topics or categories.
If you have better ideas to make it more simple but not hide it, let us know.

This is an original idea in fact you perform a subscribe way to a topic i haven't understand at first sight. We usually see a subscribe button rather but maybe your approach is the best.