I have no idea about RocketLanguages, but I can certainly suggest BunPro.
BunPro has a free trial, but it’s timed (one month), rather than being lesson/level-based like Wanikani’s, so make sure you’re ready to use it when you sign up, in order to give yourself the time to figure out whether it’s ultimately for you or not. It’ll be pointless if you sign up then don’t touch it for a month.
BunPro does have ‘Paths’ you can follow that put their lessons in the order they are in various textbooks, though it’s the same lessons regardless. They also list where you can read about the particular grammar point for each lesson, both in various textbooks as well as on websites.
If you’re going to waiting a few weeks for books and want to jump into some grammar right now, Tae Kim’s guide is a decent way to go: http://www.guidetojapanese.org/learn/. I personally used that (though I bought the printed version, it should be the same information) to get a good grounding in grammar, then filled in gaps with Genki 1 and 2. I’m now using Bunpro to reinforce and fully “learn” the things I learned from those three.
Alternatively, you could also do lessons in Bunpro and just follow their links to various resources to learn the grammar points, though I’d imagine a more holistic grammar source (like Tae Kim or Genki) would probably be better when starting out.