Without knowing the original context, topic specific permissions seems like a much broader (and as you said, much more complex) prospect than what would be required to make #4 work.
The way I would envisage this working is by extending the category permissions to include See Own. Similar to how the permissions are now, one of “See” or “See Own” must be allowed for a group that has been added.
As allowing Create automatically allows Reply, See Now would automatically allow Create - a category where you can only see your own topics would be kind of pointless if you can’t create topics.
I think this would “only” need two additional changes in permissions territory. When the groups the current user belongs to only affords See Own permission:
- The topic listing should filter to topics created by the current user
- Access to topics should be denied unless the topic was created by the current user
I suspect in reality there is additional complexity dealing with things like the Latest topics list but probably (maybe…) not an enormous amount of extra work.
We don’t currently do private support in Discourse so I don’t have first-hand experience but I think my own perceptions are in line with yours.
Discoverability is important, both in finding where to ask for support and in reviewing current and past support queries, which looks to be potentially challenging with the group PMs approach.