I’m no ActivityPub expert, but what people more knowledgeable than me have pointed out is that this results in fragmenting the Fediverse, which ends up hurting it on the long run. I’d definitely not want to be on an instance that federates with Facebook, but I also wish none of the larger ones choose to do so.
Have you tried Anytype? It’s a combination of Notion and Obsidian, and it’s end-to-end encrypted, and everything is stored on your device. Still in alpha, but it has a lot of features already. Because it’s in alpha, they only give access to people who attend an online presentation of the app. But it’s worth it. And as soon as it exits alpha, the plan is to open source it.