The videos linked in the discussion pointed to in #1 are very helpful, but primarily useful for listening comprehension. Even though examples and explanations abound, pronunciation isn't a matter of cognition. No amount of hearing and imitating the correct way to say e.g. Frühstück will ever succeed without immediate feedback. It would of course be best to have a native speaker to practice with, but an app is always available and has unlimited patience.
The sessions with sylby remind me of past practice sessions with Dragon, a speech recognition software tool, but the end results are diametrically opposed. Whereas I trained Dragon to understand and transcribe my spoken German, often despite my pronunciation, sylby is trying to train me. After hearing a sample word or sentence and recording my version, sylby provides immediate feedback. Any vowels, diphthongs, consonants, or consonant groups that were correct get marked green. What was not correct is now red and accompanied by a request to try again. Sometimes I finally get something right, but the next time it's once again wrong. A bit maddening, actually.
How have others here been successful in learning the correct pronunciation for a foreign language? Short, of course, of being born in a bilingual or multilingual family and learning as a child when the brain is more "formable".