... and not within the whole forum.kde.org?
If I find an interesting "many pages thread" and just want to know whether it contains a certain search word: how do I search just this thread?

Whatever I tried - I got results from all over forums.kde.org...

towards the bottom of the thread is a "search this topic" search box, to the left of the reply button

... you're brightly right, sorry for not having seen that.
If available, I expected this option to show up under the top-right "Tools" submenu

Tools menu only supports links and not text boxes. Unfortunately, there is only so much we can send to the top. But now you know where to find it