That's pretty much how all the old-school BBS's ran back in the day. I was running WWiV on C++ doing my own mods and compiles in the middle of the night after everyone logged in for their daily turns on the games.
The way information is now it's so much more accessible... almost dirty.
In NodeBB, subcategories are referred to as child categories. In order to set up your subforum, you would create your necessary categories (one for mobile phones, and then others for android, ios, etc) and then set the parent category of your intended subforums to be your mobile phones category.
I don't have an answer for you...
However, I did note that you are using PG. To that end, make sure to update the driver:
https://community.nodebb.org/topic/13480/redactor-error-handler-postgresql-error-on-8000-characters-message
You can view recent topics by a specific user by visiting their user profile page. Besides that you could or the users in a group and visit the group page.
Those are the only options afaik
