My intended point is that browsing all is a viable way to find communities. You won't miss many communities if you browse everything. Then subscribe to what you like, block what you really dislike.
You're right that all can be overwhelming and distasteful, but at least it really is everything, as opposed to the r/all, which was heavily filtered.
I usually sort All by top 12 or 6, depending on how recently I last checked. That's how I keep up with the general chatter. And I refresh my subscriptions by new once in a while to take part in the topics I enjoy as they emerge.
I don't think it's just federation, I believe for posts to appear in your instance's All feed then at least someone on that instance needs to be subscribed to the associated community
It might end up showing all the communities but it won't show you every post. Like F1 posts rarely end up in all despite having an active community. You're more likely to see posts from formuladank than formula 1.
The other day I got banned from A Boring [email protected] for providing verifiable historical context to the bombing of the MOVE cult. Not for taking a stance on either side of the issue, but just for fighting misinformation in the comments. I had like a 40 to 13 upvote ratio on a comment as long as a page, but the mod wants to marginalize and radicalize people so they deleted everything I posted in the thread.