How can I test my german level and see if I am actually improving? Are there any "reliable" online test that can be used? Could the "readability test" be a good idea?
Duolingo also offers the ability to take a timed test if you want. Otherwise it provides one test after another in various forms: translations, transcription (typing what you hear), and multiple choice. If you have a mic, you can even get some questions that let you practice speaking sentences. The tests get harder as you progress. You can go back and review, too.
I recommend it.
Maybe this test from the german "Goethe-Institute" is helpful. It seems to focus on grammatical details.
I would recommend reading and listening to german texts (written/spoken by natives).
The tests on Duolingo.com are very good because with use they give you a very good assessment of your current level. You can take shortcut tests to 'pass' the easier level tests to quickly get to the relevant tests for your level.