Have you looked at the Wikipedia German page: https://en.wikipedia.org/wiki/German_language#Grammar ? I'm considering basing most of my Welsh grammar study on Wikipedia, honestly. It seems to have most of the necessary information, although it's mixed heavily with jargon and can be tedious. But I find tediousness is a problem that most language textbooks share, so... It's kind of an obvious resource, but you might have avoided it, just like I have, due to its reputation.