When I had zero grammar knowledge and just a few levels of WK under my belt, I started with Grammar Lessons for Absolute Beginners by Japanese Ammo with Misa. Just watching the videos passively, I didn’t really internalize a whole lot, but it gave me a huge head start when diving into more structured study with a textbook later.
For N5-N4, I followed the Minna no Nihongo beginner series to completion, then moved to Tobira for intermediate grammar. I’m on track to finish Tobira within the next couple months, and then I’ll probably work through the Shin Kanzen Master books for advanced grammar. I like textbooks because they give exercises to help you practice, and they also have a lot of reading material that gradually scaffolds in difficulty, which also helps you practice. Personally, I prefer this to using SRS for grammar, because all of the sentences you’re reading are unique.
The main thing is forming some sort of consistent study habit. As long as you’re reading in Japanese, you’re practicing grammar. Reading is sort of a “natural SRS” in that you’ll encounter stuff that you’ve learned spaced out over time. The more grammar you know, the easier it gets to read, and then the more you’ll reinforce the stuff that you already know.
I like to pair textbook study/formal grammar study with daily immersion practice because the textbook lets me drill a specific grammar point several times in a row, which is helpful when initially learning it. I like how textbook chapters sort of let you focus in on just a handful of grammar points at a time. And if I’m reading other stuff on the side, I’ll see those grammar points come up in a more natural way, which’ll help reinforce them.
Then it basically just becomes a matter of making consistent progress in the textbook + reading in Japanese every day, and suddenly a year has passed and you’re way more fluent than you were a year ago.
Something else that has helped me has been going through A Dictionary of Basic Japanese Grammar with the book club here on the forum. I’ve fallen a bit behind because I’ve been super busy, but I’ve gotten a whole lot out of working through that book really slowly and looking for examples of the grammar in the pro wrestling fan translations I’ve been doing. It’s really cool to read about a grammar point and then find an example of it that I read weeks or months ago, and see the nuance of the grammar in action. I feel like that has done a lot to cement the grammar points in my brain.
Personally, I feel that with grammar, my understanding grows and evolves slowly over time the more of the language that I acquire. Often, when I initially try learning a grammar point, it feels a bit hazy to me, but then when I read about it again later in ADoBJG or whatever, suddenly it’s super clear to me and not hard at all. So I wouldn’t stress too much if you don’t have a perfect understanding of every grammar point you encounter.
Some people recommend cramming basic grammar and then diving straight into reading with the expectation that you’ll reinforce the grammar points that way, but personally I didn’t really like that approach. I prefer taking it a bit slower so that I can build on an already established base. But when I was still a beginner, I was also in way over my head trying to do a hugely overambitious translation project, so I think following a textbook alongside that sort of helped ground me and give me a sense of actual progression.
I guess my advice mostly boils down to “do consistent practice” and “trust the process” 
. If you’re engaging with Japanese, you will be getting grammar practice. And if you get enough practice, eventually you will learn. You just have to find a method and a schedule that you are able to keep up with and do every day.