It’s also important to remember that WaniKani is useful to help you learn 2000+ kanji, a number too big to memorise by brute force. For the early chapters of textbooks however, you can just memorise kanji meanings and readings as you encounter them.
and so on
You absolutely can learn these words outside of WaniKani. It’s fairly easy to memorise - or at least recognise - some 100 or 200 common kanji, which is more than enough for basic grammar. Plus, don’t beat yourself up if they don’t stick too well to your memory, WaniKani will teach them again later. As a bonus, lessons and reviews are way easier for items you were already familiar with.
Basic textbooks do not assume you’re doing WaniKani, so you should be fine.