I recommend you give Tae Kim’s Grammar Guide a shot. It’s free. (I’ll address Genki exclusively eventually, I promise. Please skip to the Genki heading below if that’s all you’re looking for.)
I have used Genki in formal (classroom) education and have otherwise referred to and reviewed it on and off for years. I don’t take issue with its content. After reading Tae Kim, though, his work helped me put into words what I felt about Genki in general. I don’t much prefer Genki’s pedagogy. Genki is written in many parts to teach Japanese in ways that fit more easily into the English-thinking mind. In this way, it’s approachable in schools and easy to build a curriculum around. I didn’t realize this was suboptimal at the time because I didn’t realize there was any other way of approaching Japanese language-learning from a textbook, was naive enough to think that the meaning of some phrase X in one language can always be sufficiently captured in a translated phrase Y in another language.
I find that Tae Kim’s work puts the learner in more of a Japanese-language mentality (vs. English-language) when learning. You’ll read things translated like, “I read book,” which sound silly to native English speakers. There’s a very good reason why things are translated this way, though, and Tae Kim explains why he often leaves out indefinite articles, as one example, in translations. I find Tae Kim’s presentation helps me grok grammar more easily than Genki’s presentation.
Tae Kim presents grammar in a slightly different order than it is presented in Genki, too, and I find Kim’s ordering to prepare me more efficiently to parse increasingly-complex Japanese sentences.
A Word on English
I don’t mean to presume that your native language is English. My comparison of English-language vs. more Japanese-language-like mentalities is generalizable. All I’m getting at is that I find Genki’s approach more along the lines of, “you’re already good at <language that’s not Japanese>, so here’s how you can think about Japanese in <language that’s not Japanese>,” which may train you to form Japanese thoughts by mapping from <language that’s not Japanese> instead of training you to think more like a native Japanese speaker earlier on.
Okay, at long last, addressing your statements and questions about Genki itself directly in case you have no interest in Tae Kim or alternatives (no worries, use whatever tool works for you)!
Firstly, to be clear (since I mentioned earlier that I don’t prefer Genki), Genki is not bad. Once upon a day, I traveled around Japan using grammar learned from Genki alone. It works.
I’m just not retaining a lot of the kanji and gramma with [lingodeer]
This will apply to Genki in ways, too. SRS is the key to long-term retention here. Make flashcards, sentence cards, etc., whatever is your weapon of choice.
I have no idea how to efficiently use genki alongside wanikani
In my opinion, there is no such natural way, only artificial (your style fit to your tastes). Here is what my style was (post formal education). Adjust ingredients to taste, of course.
For any given chapter:
- Read the intro skit. You won’t understand some parts, no worries.
- Read the list of new vocab. Slap that ish into an SRS. Do not skip the SRS part. (PS: Vocab out of context can get very boring…)
- Review your new SRS material. (This is optional if you proceeded from Step 2.)
- Read one or two grammar points.
- Skip to the exercises and work through the exercises pertaining to the grammar points you just learned.
- If you have the workbook, do the associated workbook exercises, too.
- (Next day) Optionally go to step 1, skip step 2, and then proceed starting with step 3. When you finish the chapter, go back to step 1.
Often associated with Genki are materials such as Kanji Look and Learn. I recommend you don’t even bother with these things. WaniKani and general SRS are leaps and bounds more effective. If you find kanji in Genki that you haven’t learned yet in WaniKani and that you want to start reviewing, either use user scripts for WaniKani to access reviews for such an item or create that item in whatever SRS you use for vocab.
Finally, I found Genki most effective when worked through with a partner with whom I could speak in Japanese and do the pair work.
You mention you’ll be using Genki for about two hours every day. At this pace with the above steps, you’ll complete at least one chapter per week, maybe two, depending on what your completion criteria are (ex: mastered, whatever you choose that to mean, all vocab in the chapter in your SRS). Assuming one chapter per week, that’s about 12 weeks per volume, which would put you at about 24 weeks (6 months) for both volumes, if memory serves.
Regarding grammar not sticking, I find this to be the general case for me to if I am not reinforcing my grammar lessons with interesting input. Read what you enjoy and are generally capable of understanding (manga, magazines, books, websites, etc.), watch and listen similarly.
Best wishes to you on your search for the perfect blend of material and methodology in your Japanese language-learning, finding what works for you! Keep at it! As others have said, do check out the Genki reading group if you’re going to stick with Genki.