Charlotte Link writes pretty good psychological thrillers that deal with psychological depths without falling into kitsch. Her books are rather dark and realistic. Then there's Nele Neuhaus, who also tackles complex psychological conflicts in her crime novels - not exactly self-help, but mental health themes are always present.
For more factual approaches: Bas Kast has written "The Nutrition Compass" and other works that also address psychological well-being without getting esoteric. Giulia Enders ("Gut Health") also treats the connection between physical and mental health quite informatively. And then there are authors like Stefanie Stahl ("The Child in You") who work more in the therapeutic realm - it's somewhat self-help-y at times, but way less kitsch than some English-language works.
For fiction with substance, I'd also look at German-language authors who write about their own experiences. Sometimes smaller indie authors are more honest and unpolished than the big names - just harder to find. Maybe checking Goodreads with filters for German books and the tag "Mental Health" would help too.