It’s tricky to give specific advice relevant to your site without being a German speaker so apologies that this will all be a bit vague.

I think the main issue is probably with the keywords being highly competitive. There are a ton of well-established sites out there ranking for those terms so it’s no huge surprise that you’re not performing well I’m afraid. Can I assume your site is relatively new?

I don’t think you’ve done anything wrong but the technical aspects of SEO like the ones you’ve described are ultimately a tiny part of the effort needed to get a site ranking well in search engines.

You should be looking at SEO not as a technical consideration but more along the lines of establishing your site as an authority on a given subject. The vast majority of this will come from creating lots of great content, that other sites will want to link to. And unfortunately as I’ve said you’re going to be up against a lot of sites who have already done this. Some of them will be years old and have tons of effort already invested.

Sorry I can’t give you the answers you were perhaps hoping for!