As long as you’re unsure whether you really want to commit to learning Japanese there’s no harm in some Duolingo. But it’s getting useless really fast for the aforementioned reasons.
That said, to get a basic feel for the grammar I would recommend the Japanese from Zero video series. Be aware that in order to make real progress you will need to actually practice, just watching the videos will give a false sense of of how much you really know. The videos are free, but they are accompanying a (commercial) textbook series for self-studying. I haven’t tried the books, but judging from the videos they are moving at a glacial speed 
Otoh, the popular Genki workbook unfortunately relies heavily on teamwork and does by default not offer an answer key which is crucial. Also it’s a bit expensive. Unless you’re a citizen of Russia, in this case you can legally download a copy for educational purposes from the library genesis web page.
Last bit of advice: I started out on Genki and Tae Kim as a non-native English speaker and while I have a good command of the language I underestimated how much easier it became after I switched to resources in my native language. In case you’re German I highly recommend https://www.japanisch-grund-und-intensivkurs.de/ before any of the classic resources mentioned around the English-speaking net.
I did a lot more anki flashcards than I did with my first language though
And even though I went into this with the mindset that I’d just learn grammar by immersion like with English or Swedish, I quickly gave up on that because the grammar is so different, meaning I had zero intuition to work with.
Going through Tae Kim a couple of months into my studies (when the aforementioned frustration set in) helped my reading immensely.
Can’t say I’m much of a believer in learning other languages like we learn our first one, really.
I am using Tae Kims guide and I already understand past negatives and such. Thanks again!
In case you are not aware already:
文プロ(Bunpro) : Update 6/13 - New N1 Grammar - Japanese Grammar SRS Study Site - #2900 by pushindawood
Overall I wouldn’t recommend Duolingo for grammar, but it is useful for getting the basics and people frequently talk about the different ways to say/write things in the forums, along with cultural information. The course is getting revamped and the mods/admins work to constantly improve it. In fact, if you make a new Duo account, you have a 50/50 chance of getting the 4.0 version of the course (A/B testing), which is focusing on a lot of practical content and expanding the scope of the course.
I like Tae Kim, but even with using just Tae Kim and Wanikani, you’d have that gap of how people talk in actually. Stuff like common phrasings and how people shorten things. Currently I live in Japan, so I’m getting this learning in just by listening to people around me and therefore I don’t have more resource recommendations.
Fluency is hard work. Best of luck to you in persevering!
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