Why not Multisite? Running a separate discourse database and front end within the same footprint? It’s going to be much easier to support a partition at that level than it is to embark on any of the above.
When you get to the point that you’re separating the theme, front end and content you’re effectively talking about separate Discourse instances. That’s pretty much why multisite exists. The added benefit is that they can make use of different site settings and sign-on.
New plugins and code changes aren’t just adding stuff to accommodate a new forum with the above, you’re adding complexity and a big risk of blowback onto your existing customers.