The currently common older implementation of e2ee in xmpp has the same issue with only the message body being encrypted. There are newer specs of OMEMO that have better metadata protection, but its adoption in xmpp clients has been very slow.
Prosody is more of a sandbox, with Snikket being a preconfigured version of it, but yes running Slidge will be a bit easier with a normal Prosody server.
Then you have not enabled to full image proxy (and note that it does not work retroactively). Here on our instance all the Nicole images were proxied correctly to protect the privacy of our members.
But there are some efforts to add multi-user video calls to full xmpp clients as well. Dino can already do it for a while, and Movim and Libervia recently added experimental support.
Its not quite a full Discord replacement, but for private groups it works quite well.
Yes, in theory, but in praxis no because self-hosting the sync server alone still depends on the centralized auth server from Mozilla, and self-hosting that as well is possible but complicated. It's sadly a mess, and you might be better off not using Firefox sync at all.
As for your other question, depends on the specific addon, but usually no.
You still need to log into their servers and thus provide them a lot of meta-data, like IP addresses, when and approximatly where you are using your browser, on how many devices etc.
First of all that's all hypothetical and secondly that thought experiment only talks about an app that uses the PDS as an auth source and data storage. It does not talk about your PDS communicating with another PDS, for which AFAIK a relay is needed.
Hosting your own PDS would be like hosting your own email, but with the caveat that you can only access it through the Gmail interface and need to use the Gmail relay to communicate with others. In other words: completely pointless.
Mostly yes, but people were a bit naive back then and many onboarded friends and family onto the Gmail service and that burned some bridges and good-will as for Gmail users other xmpp users simply ended up as if offline and never responding.
This is not what really happend. Yes initially they added features other XMPP implementations had trouble to catch up with, but the main problem was them not implementing important security features like s2s TLS encryption, thus forcing others to cut them off. Google continued to run their xmpp servers for many years after, but they were so badly maintained and insecure no one wanted to interact with them anymore.
The rest of the xmpp ecosystem continued to grow at a slow pace and is alive and well, it was just an annoying set back going from being able to contact many millions of users on their Gmail linked xmpp accounts to not being able to do that anymore.
Maybe it was based on the "lifetime" of their hamster 🤷