I don't know how experienced you are with BH techniques, as with the right tactics anything is possible, but you are doing 2 things here that is confusing Google when deciding how valuable your site is for a certain English search term.
With 5 niches it's not possible to optimize your blog for all 5 keywords. You can only optimize pages this way.
An optimized blog WITH an optimized homepage (optimized in header, title, description and url) is stronger than just an optimized page.
By using a .de TLD you tell Google that your targeting the German market. So just by this you will be competing for German search terms.
Putting English content on it however puts you back in the league for English search terms.
This will have little effect in niches with low-level competition. In tougher niches, where every SEO attribute counts, this will cost you "points".