if you've made it through fitzek, i'd really recommend checking out valentina cebotari's books. she writes psychological thrillers that genuinely won't let you go - similar to fitzek, but with a slightly different style. her stories are often set in eastern europe and have this dark atmosphere that really gets under your skin.
another direction entirely is juli zeh's books. not exactly classic crime novels, but the psychological tension is brutal. "leere herzen" for example - you're never sure the whole time whether the characters are insane or if there's actually something real going on.
and if you're willing to read in english: louise penny writes canadian crime novels with a detective named armand gamache. the books build on each other, but you can also read individual ones. they're extremely well thought out and not as off-the-wall as fitzek, but they're intellectually really satisfying instead. perfect for switching off, because you completely disappear into the world.