Can you see which communities I follow?

Wouldn't need to see it directly. If someone was to tag enough posts they could deduce it over time. Eg, I could post on every community on every lemmy in the fediverse and over time I can be reasonably sure which communities you follow as you'd see these post in your feed and tracking images would populate your view of them as you scrolled. Would take very little automation to do it.

Which feeds I watch (and when I do that)?

Yes.. because it's possible to use "normal" images to track who's downloading those images, what addresses/user agent/referrers over time is powerful. After enough time, it's entirely possible to deduce which feeds/communities you're watching. Eg, if I post 10 different items, and 3 of them come back to your specific IP address, I would have a really good estimate on which feeds you're likely on. Do this at scale and I bet you could deduce it completely and probably with much less time and hassle than you're thinking. Hell because of my reverse proxy I can see EVERYONE who loads my profile picture. I see ALL the users to run into my posts on complete fucking accident. Lemmy loads /inbox to pull that data.

Hell this is the core reason why everyone pushes back on 3rd party cookies these days. It made this tracking trivial. Tagging every page with some image or asset that forces a connection is effectively the same thing.

Who I interact with through DMs?

I've already stated clearly that this would be the hardest thing. Just because there's one or 2 things that would be hard or impossible to obtain (even over time) passively or as a complete outsider doesn't make the rest of the argument wrong. All it would take is either site operator to leak the data, any type of MITM, etc... to leak the plaintext content of your DMs. Hell federation leaks where it sends data outside of the expected subscribers has happened. Then you have to also realize that many instances use services like Cloudflare or other WAF solutions to stop DDOS's and such.... Those nodes can read the plaintext DMs and all federation data. Any malicious actor that manages to break any single part of the chain has access to it all... and it can be quite trivial in many instances to do so.

The Lemmy system is not "secure". It's not meant to be. Everything on the fediverse is public and all of your actions here are trackable by many parties in many ways even outside of the operators of both ends of the federation action itself. Including how you're connecting and using the system.

DMs alone, and actual hashed passwords are not really needed for a third party threat to act malicious and get all of the aggregated data they'd ever want. You pointed out specifics, I answered those specifics. Then you pivoted to other shit that I ALREADY outlined. This argument is super disingenuous.