To launch the discourse hub app, the registered URL is discourse://. Put that into LCP. I haven’t seen any additional parameters for it to say jump into a particular forum, but that’s not to say there aren’t any.
The Discourse developer’s approach to documentation is more just in time… which has the disadvantage of leaving gaps and not providing a structured way to find relevant information.
You currently can’t have nested groups or additional spots; without getting a bigger screen. What you can do currently is use lists to put multiple action options onto what would be a single action entry (see earlier in this thread). Note that lists can be nested (see my example here).
I did see reference on Twitter from the devs to a planned change in the way to access action groups that makes me suspect something else is on the way that may allow for more launch options. At the very least it will pave the wave for an option to expand.
Hope that helps.