Some people are good at learning grammar from a textbook or a web site such as Tae Kim’s. Others (such as myself) find that route doesn’t work very well. And if you have limited time and energy to study/learn each day, that might make it more difficult.
One path to follow would be to pick one of the previously read books from one of the easier WaniKani community book clubs. You can read through at your own pace, you’d have available discussion threads where grammar is talked about to help learn grammar in context, and if you have questions about material that didn’t get covered, there’s a good chance some people still have the discussion threads on notification and will answer questions.
Granted, most of the items read at the Absolute Beginner Club and Beginner Club levels are manga, but you can find some fiction and non-fiction books too. In the end, though, the best pick to read will be something that interests you personally.