Would a single-user self-hosted frontend for interacting with Fediverse apps be feasible?

Hello!

While discussing about privacy on Lemmy and in the Fediverse, I've stumbled upon an idea that would solve some of the issues inherent to the fact that you need to have a home instance, that is under control of someone you have to trust. But my knowledge about ActivityPub is lacking, and I'm not sure if something like this would be possible or not. Also - it possible that something like that already exists, but I didn't manage to find anything.

So, would it be possible to create a Fediverse/ActivityPub app that is just a self-hosted frontend for interacting with other apps, such as Lemmy or Mastodon, that only hosts your own personal data related to your account, but not the content you post to other instances?

The main thing I'm unsure with is how Fediverse works in this regard - who hosts the content. If my home instance is programming.dev, and I create a Post or a Comment on lemmy.ml, who is the source of truth for that post? Does the content get saved on my home instance, and Lemmy.ml only gets an ID that it queries if an user requests it, or do I send the content to Lemmy.ml to live on their server?

Depending on this, it would make such a self-hosted app easier or harder. If the content lives on the instance I post it to, it would mean that you can create a fediverse app that only stores your personal user information and DMs, and you don't have to deal with serving your posts to others - because they live on the other instance you posted it to. Then all that would be left is to create an UI for displaying and querying content from other instances, and you have a way how to interact with the Fediverse without risking any of your personal private data.

On the other hand, if the content would have to live on my instance, I would have to deal with serving it to whoever requests it, which would make it a lot harder to self-host.

I kind of hope it's the first option, because then it would allow for public communities of content-only servers while also letting users have their own personal-data only instances that allows them to interact with the rest. And I really like that idea, because it would allow you to for example have reliable E2E for messages, since you have the code that generates and stores the private certificate under absolute control, and only need to share your private key with others.

In general, it seems like a great solution to many privacy problems on the Fediverse, and if something like that would be possible (without having to serve the content, because then it may get too resource-intensive for a regular user), I would definitely try to come up with such a solution.

And now that I think about it - if you actually have to host the content, then it maybe be possible to create a combination of user-data / content servers, where you select a public community run content server to host your data, and have the personal user-data server self-hosted. And if a request comes to your user-data server for content, you just redirect it to the community-ran server. But that's just brainstorming.