It would be great if this worked with all themes and not just block themes. Looking at the block world, I would consider using Kadence theme but that route doesn’t afford access to all the FSE goodness yet, as far as I know. Why Kadence? It offers a really good header and footer and albeit one that exists in the old customiser UI, it is more robust and easier to use then what the purely block version offers. Again, this is the downfall off the Gutenberg project. It’s too rigidly stuck to it’s way or the highway and not built to bring everybody along fot the ride. All the great page builders out there are left to hack their way into the space without been given developer hook to build on top of the block editor.