You definitely need to combine them, but it's better to think of them not as competitors but as different stages of one process. Duolingo is great for training muscle memory - you learn words, you see patterns, without overthinking. But when you later encounter an actual German text where grammar works in context, there can be gaps. A textbook fills those gaps: you figure out the why, not just the how.
My recommendation is to use Duolingo as your foundation for daily 15-20 minutes (it really keeps you in shape), and pick up a textbook 2-3 times a week when you want to understand a specific rule that just "flew by" in the app. For example, if you came across the dative case in Duolingo and just memorized the phrase but didn't get the logic - open the textbook, read the explanation, do the exercises. That's what works.
One more thing: the fun you get from the app isn't the enemy of learning, it's the helper. You've already been studying the language for six months, so you have motivation. The key thing now is not to burn out. If a textbook feels like boring busywork, putting it aside and sticking to Duolingo is still better than abandoning the language altogether.