Hmm, that actually sounds like a great idea. Does it actually need to be reachable from the outside, if you don't want to host any of your own communities on it? Or will it be enough for the instance to just pool data? Apart from no-one being able to contact you via DM, that is.
I'll look into it, having my own home instance actually sounds pretty easy and it may work.
Actually - wouldn't it even be possible to build a browser extension for that? One that just simulates ActivityPub calls, and you just browse on someone else's instance without logging in while still allowing you to comment or vote on your behalf?
EDIT: I've posted some more thoughs about it to another comment, which I assumed was a reply to this one. The more I think about it, the more I really like the idea of a self-hosted front-end for Fediverse apps that doesn't host communities, but only user interactions and allows you to interact with other apps and instances.