Personally, I believe Tofugu is wrong and it’s silly to wait until level 10 to start studying grammar. I think the best way is to be learning both vocabulary and grammar as soon as possible, and most importantly I think the most essential part of language learning is to immerse yourself in the language. Listen to a lot of Japanese stuff without subtitles, and eventually do a bunch of reading too.
You don’t have to understand everything, because understanding comes gradually. Eventually your brain will start picking up on patterns you’ve learned when studying grammar and vocabulary. The way children learn language is actually the best way for adults to learn language too. Even if you don’t understand at first, by being immersed in it constantly your brain will eventually start picking up on things in a way no school-course can ever teach you.
I also think most grammar resources are honestly quite horrendous. Genki is a textbook for school, meant to be used in a setting where you have a teacher and coursework. It’s not appropriate for self-learners. Apps and stuff like Japanese from Zero are all usually really bad at trying to speed you along by overly relying on English grammar theory in ways that just do not suit Japanese, leading you to end up confused and stuck further down the line. JapanesePod101 is just generally full of misinformation and bad advice.
In my years of trying to learn Japanese, I’ve found two grammar resources that are actually really good at teacing grammar from a Japanese point of view. It’s TaeKim’s guide to Japanese, and CureDolly on YouTube. CureDolly especially is really, really good at teaching Japanese grammar in an understandable way. Almost every learner who gives CureDolly a shot comes away with the sense that they finally understand the things all the other grammar resources never managed to explain. The only issue is that her voice changer voice can be a little grating and hard to get used to, but it’s well worth it to power through.
Or alternatively, here’s a list to someone who’s made a transcript of her main playlist videos: CureDolly transcript. Personally I can recommend a mix, where you first watch the video, with CC, and then come back to read the transcript to make sure you really take in the lesson.
Either way, good luck with your grammar studies! I really do recommend that you start as soon as possible, and put a lot of time into it from now on. I also recommend you find some Japanese entertainment to watch without subtitles, to give yourself more of that language immersion time.
EDIT: Don’t mind my WaniKani level btw, lol. I recently reset my progress after I left off at level 18 years ago. I was stagnating with my Anki deck because complicated kanji’s were hard to parse, so I figured I wanted to give the WaniKani radical-based approach another go. Hoping it might make it easier to differentiate the more complex kanji when I can separate them into recognizable radicals!