This sort of thing has been talked about many times before. Everything you do publicly (post, comment, and even vote) on lemmy is federated to every instance where a user has subscribed to where the community is hosted, meaning possibly thousands of servers. Once federation happens you have no control over it. You can "delete" it, but that just sends a federated message to those same servers saying "this user wanted to delete that post". There is no guarantee they honor that request or mechanism to force compliance. Servers could even pretend to delete it, but just hide it and keep a copy forever if they wanted.

The internet is forever, especially in the fediverse. If you post anywhere on the internet someone can make an archive link that shows the post, your username, and any media forever.

Deleted comments remain on the server but hidden to non-admins, the username remains visible

This makes a lot of sense to keep a copy of what was deleted in case it is needed for all manner of things such as reporting to police or validating a pattern of a user's bad behavior even if they are deleting their messages.

The username thing is potentially a problem, but I think is just part of the federation protocol. The protocol has to identify who is doing the delete (so not just anybody can delete anyone else's comment), and that delete message basically takes the place of the deteled content. There may be a better way of handling that specifically in lemmy or the UI, but there is no guarantee other servers will behave the same, that is information that is out there.

Deleted account usernames remain visible too

Basically the same as above.

Anything remains visible on federated servers!

By design.

When you delete your account, media does not get deleted on any server

Not sure about this, there are layers of caching involved and those do get auto purged by default after a maximum of 6 months or something. This is a pretty common thing where if someone posts something to social media you can find a CDN link that will keep working for e.g. a photo even if the post holding the photo is deleted. It could probably be improved by servers finding and purging that item from their cache when they get a delete message.

the operator and developer of lemmy has very problematic politics

This whole section is basically fear-mongering. Lemmy is just one implementation of a federating server and these "problems" have basically always existed. Yes, the developer of Lemmy is a communist. If that causes mistrust don't use lemmy.ml (the only server the dev runs AFAIK). If you don't trust the software written by a communist for some reason, then use Kbin (if you want a reddit-like view of the fediverse) or Mastodon/Pleroma/Akkoma/Misskey/GNU Social/PeerTube/Pixelfed/Friendica/etc (various twitter/facebook/youtube/instagram styled fediverse softwares).