010101 you can create dynamic pages that don't use the Flarum SPA in Flarum, those make use of the basic blade template https://github.com/flarum/core/blob/master/views/layouts/basic.blade.php It's what the 404 or 403 pages use right now, but there's a plan to move those to the SPA so this template might not exist in future Flarum versions.
However such pages don't have the header with session dropdown and such. To have the header, you will need to boot at least part of the SPA.
You can also create pages that render from your own blade template, or even from another templating engine. Anything that can be returned as a ResponseInterface, Blade\View or string can be used. But you would have to fetch the Flarum information (like session) yourself and then also write how to display it on the page.
Technically you could create a standard Flarum page and not mount any Mithril component, but it might not play nice with the navigation from/to other pages which could expect it to be part of the SPA as well.
While what you describe is technically possible, I don't think it will solve the problems you're trying to solve with it. As soon as you load anything from Flarum, you're dependent on our unstable API. And by extending Flarum in a way that's not intended (dynamic PHP vs Mithril SPA), your use case will probably not be covered as extensively by our update guides.
