Definitely agree with Human Japanese as a great, gentle intro to grammar. I’ve been peppering a chapter here and there into my studies and really like the down-to-earth explanations.
I like the Duolingo English-to-Japanese course. It’s not perfect and I wouldn’t recommend it as your sole learning source, but I like the option to turn off their word pickboxes. That way, you’re forced to type in the answer yourself, which I find to be quite good practice after you’ve already picked up your grammar lessons from elsewhere.
Also, a book I recently picked up that I really liked is “Unlocking Japanese: Making Japanese as simple as it really is.” The author also has a site where she has videos and articles discussing various grammar points. I recommend it once you’ve dug into some grammar basics, but there were some insights in that book that completely changed the way I approach and understand the language.