When I started with OH2 a month ago, I just installed the binding through the PaperUI interface.
Nothing else was required (besides the configuration file of course), and all my mosquitto traffic works perfectly.
This may sound dumb, but I did as you did, nekmech: I installed the MQTT binding through the Paper UI. It seemed to go well, but I can’t find a way to configure things through the Paper UI. The Astro binding worked out of the box, as did the Tesla binding (not that I have one yet haha), but the MQTT binding doesn’t show up on the binding configuration page.
Indeed, my instructions are not needed anymore, as by now there Is the easier route via paperui.(If everything fails, you may use it as fallback)
Suggest to switch on logging in karaf, than you can see what happens when mqtt binding and transport are being installed. … and you need the cfg file as described above.
You are best to install the 1.9.0 snapshot via the Paper UI and you will need to delete the earlier versions from the addons directory. I would do this first and then restart OH2 prior to adding the 1.9.0 binding via the Paper UI.
After that follow @nekmech 's instruction above and then create your items in an item file.