It’s all down to how you learn. I personally consider the mnemonics to be the worst part of WaniKani. >.> I am a terrible visual learner and extreme audio learner, so the “incorrect” mnemonics that WaniKani uses more often cause me to get things wrong than right (for example, 作 is さく but is given the mnemonic SUCK [instead of SOCK], while 足, pronounced そく, is given SOCK [instead of, say SOAK], resulting in my constantly typing さく when I first came across it since I only knew the reading あし at the time). Someone who is much less reliant on audio likely wouldn’t be so stuck on those issues like I was.
Having now been living in Japan, I’m finding using radicals and shapes to be much more useful a skill when trying to learn Kanji outside of Wanikani. When I receive my electric bill filled with a ton of kanji WaniKani isn’t going to teach me for likely more than 30 levels from where I currently am, being able to identify radicals and use Jisho.org to look up the Kanji is significantly more useful. Unfortunately, as a pictographic language, mnemonics will only get you so far, especially as more and more kanji start revealing additional readings that the WaniKani system just throws on you after you’ve drilled on reading for several levels and tens of vocabulary.
I think the most important detail of your original post was your intention to learn how to WRITE, the skill most non-native Japanese speakers lack the most. I think your approach to using radical and shape identification will be very useful for that. Use Jisho and Wiktionary to learn the stroke order of the radicals you learn while you are waiting on reviews and your visual memory will easily be able to keep up with the SRS (at least to level 10). I still feel like the SRS is way too slow.
As for what you’ll be missing if you only use WaniKani: grammar. No matter how many characters you learn to read and write, I can guarantee speaking at any reasonable pace will be a serious pain if you don’t use some type of grammar supplement. As for what you use, I won’t give a learning supplement recommendation, as I think all of them (Genki, Tae Kim’s guide, etc) are all equal, as they all fail to provide any worthwhile method of grammar application in the listening/speaking department, and, with proper attention to listening/speaking exercise, they all cover what you need to know. Whether they are “boring” or not all comes down to your personal learning style and how much investment you have in learning the language (learning for fun vs learning because you like the language vs learning to be able to read manga/light novels/watch anime raw vs learning in order to work and live in Japan without difficulty).
Whatever reference material you decide to use for grammar, definitely try to regularly expose yourself to native Japanese speech (anime doesn’t count for this, as grammar is either textbook or anime, and speech is intentionally more articulate and rhythmic) such as Japanese radio stations, TV shows (the ones with spokesman and conversations, not dramas and such for same reason as anime), and do your best to try to recite to yourself what you hear. That way you can get some “muscle memory” when speaking and get a feel for how native speakers positions words, phrases, and pauses.