If you like opening all your documents in a single instance of Kate, replace the command line “kate %U” used in KDE’s file binding by “kate -u %U”. This is actually quite well documented in Kate’s manpage as well as on the faq on the homepage, yet somehow I thought it is useful to note this one down as it is not accessible from the GUI.
You can find KDE’s file bindings here: control center > KDE components -> file associations. From then on, every file you like to open with kate through the context menu will use those settings, even when it’s not a txt.