Grammar vs Kanji vs Radicals

I have downloaded the "Learning Japanese" App, got access to copies of the Genki course and the Kanji Books (both basic and intermediate volumes) and am finally confident in my Kana reading ability.

I have virtually no vocabulary, a very limited idea about the structure of the japanese sentence and little useful knowledge when it comes to grammar.
So ... where do I start?
I could work through the App, which - after a quick first scan - seems to be rather neatly structured.
I could ignore the app and instead work on the Genki course that I have heard but positive remarks about.
I could also put grammar aside at first and work through the Kanji book(s), which undoubtedly would help me in working with either of the above.
Or I could delay that as well and instead memorize the kanji radicals which I have heard should greatly help in learning the kanji in the first place.
Personally, I think the
Radicals -> Basic Kanji Books -> App -> Intermediate Kanji Books -> ???
approach would make sense.
But since I am a beginner here, I thought it may not be such a bad idea to ask somebody who is not.
What's your thoughts about this?

I'm not familiar with the app or the course you're talking about, but my general advice would be as follows:

Start off with a book (or an app, if there is one) that does grammar and words at the same time. You could memorize words and learn grammatical structures one and one, but I think you'll find that pretty difficult to put in use. Japanese grammar is very difficult from most (specifically western) languages, and the more you read and practice - the better. Personally I think みんなのにほんご is great, and it's what I used when I started learning.

When you're starting to get the hang of that, of course you can learn more words than what are included in the book.

For kanji, I advice you to get a book (now I understand you have some already, so if they don't match I get that you wouldn't want to buy new ones) that co-teaches radicals as well as basic kanji. You'll find that most kanji for the basic level (JLPT N5) are the basic radicals. With the right book they'll explain the radicals, the difference between similar ones, and the meanings. For me, this made it a lot easier to memorize. And even now (studying for N2) knowing the meaning behind the radicals makes it easier to learn new words.

You really won't need kanji at first, and I advice you to start with grammar and vocabulary. Then you can work your kanji into that. If you're like me and love learning kanji, just try to push it in as much as usual. If you want to be able to speak and communicate in basic Japanese though, the kanji are honestly the least important.

I hope this helped! Feel free to ask if you have any more questions!

I would start working through Genki or the app you mention (presuming it is similar in content to Genki) but you will probably find that they don't introduce many kanji to begin with. If you want to get a head start on the kanji, you could split your time between Genki and your kanji books. Generally speaking, you don't want to be learning lots of grammar but have no vocab/kanji skills, or learn a huge list of words but not know any grammar.

There's no need to memorise all the radicals or to know their Japanese names. It's a good idea to understand about the radicals (and also non-radical components), and how they are used to make up kanji (phonetic components, etc) and to recognise some of the most common ones, though.