While I have been very busy rearranging my life, I haven't put much effort into the thread since it seems very dead compared to the last one. That's why I wanted to try to do some community topics/discussion/activities but it's hard to gauge how many people even frequent this thread at all if they don't post. My apologies for that. It's probably why I'm taking the time to reply to this post..so sorry for the length.
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I think when I wrote that post, the previous thread and previous community of posters were sort of stuck in a funk. A lot of people basically at the usual hump most language learners get to. You run through the basics, the textbooks, and then you're kind of left swimming in the ocean alone without a clue where to go.
The origin of the whiteboard method for me came from sort of a similar place. I realized that if I continued on the path of a university education's curriculum, after years of "study," I would be at a very low level - even if my school was ivy league. For me, I just identified the existence of the hump before I got there.
Also, since I wrote that post/guide with those specific posters at the time in my mind, I connected the whiteboard method to the other parts of the JLPT study like the grammar/idioms. I didn't have complete beginners in mind, but it doesn't mean the core of the method can't be adapted to fit any level's needs. The method is just brute force memorization through repetition and muscle memory. The act of carving the concepts into your body through your hand is what locks it in. The picture memory you build from staring at those lists for hours never goes away.
I guess what you really want to know is can you juggle/balance learning from a structured beginner's textbook while doing some crazy self-study method at the same time and how will they help or interfere with each other. It probably doesn't feel natural to have these two wildly different study methods with different content going on at the same time.
Even if you begin with the JLPT lists, you're not sure when the content will overlap with the genki books and if you'll be repeating a lot of study. In a perfect world, you would do the whiteboard method for the first two JLPT levels and that would probably ensure that any vocab you encounter in genki will be already known. Basically, you would make genki easier/faster because you don't have to waste time with vocab.. but in essence this sort of makes genki useless because at that point, why not just do the method for the grammar points at the same level as genki and ignore genki altogether?
I guess the answer to this is more about how much you want your hand held (not in a bad way) as you develop the foundation of the language. Is a grammar point, dictionary-style explanation/definition, and a few example sentences enough for you? Or do you want to spend a chapter or two explaining a topic and slowly build up using whatever methods the textbook does.
At this point. since you have the textbook coming, work your way through a bit of it first and see how the language feels. I mean, if you have the time to use the whiteboard method, the truth is, you could complete a textbook like genki in probably 2-3 days. I don't know the contents of the book, but very randomly I would assume it does stuff like polite/dictionary form, present/past tense, negatives, u/ru verbs or i/na adjectives, particles, te form, and maybe random stuff like nagara/kudasai/ka.. whatever.
There's something about textbook study that makes these concepts feel way more epic than they really are. Like in your mind you feel wowwww.. in the beginning it was just desu and now 10 chapters later I have no clue what all this kunakatta stuff is. While with something like whiteboard, the most basic concept of desu has the same weight as learning something like a conditional. It's just another point on the list, not a build up to anything. The grammar points I studied from N1 lists feel the exact same as those from an N4 list.
I think that goes into a very different topic of the psychology behind study and study methods. So the very basic TLDR I would write is to give genki an honest shot and don't worry about the whiteboard being a supplement right now. At the core of the whiteboard method is a repetition of any content you have already studied because of starting from the lowest JLPT level and working up. So whether you start alongside genki or after, you will be writing the most basic words/grammar points multiple times either way.