I don’t think “siloizing” your Discourse like that would bring any benefit.
Have a look at our site (forums.jag-lovers.com). We have categories for each specific model family, and common categories. Our site has always been organized this way. I see more “cross pollination” between the categories since we changed to Discourse - on the old system there was a bit of a “circle the wagons” mentality. Discourse’s search, latest topics presentation, and the ease with which users can link to posts in other threads are the only UI changes, and our user base is more or less the same, but people are helping each other across “knowledge domains” more than they used to.
The only benefit from multiple instances would be if you wanted to have category/instance specific moderators… but that means more staff/effort/duplication of moderator tasks.
You could use sub categories (Bragg-schooling, Bragg-social, Bragg-offsite) for base specific content and have a few generic categories (Army Life-Army Wife, Military parenting, hobbies).
Set up a user custom field “Current Posting:” for your users to indicate where they are, and get them to populate “About me” with past postings, and display that on their user cards.
You probably won’t need to move many things around because users post in the “wrong” category.
Using one instance will decrease your moderation effort.