Chess tactics and Chess King questions

I'm trying to start getting serious about chess study. Right now my rapid rating is hovering aroung high 1300s to just 1400. My tactics rating fluctuates between 1850 and 1950 (though I have hit 2000 and went back down).

So my main question right now is about tactics training. I'm trying to find out how I can build my tactics strength. I want to use a tactics training method where I complete a book/course and repeat it a few times before moving on to harder tactics. So how should I start and progress? 

I've heard a lot of good things about Chess King tactics apps and courses. If I were to use those, and only those, would that be enough until I reached like 2000 rapid (not tactics)? I went to the chess king website, and it shows a course that has 4000 tactics and some others that have 1200 and so on. That seems like quite a large amount and they seem to have progressing difficulty, so would there be any benefit to using other chess tactics books? This is the website I'd purchase the courses from

Also, the beginner Chess King tactics courses are puzzles aimed at 600-1200 players, so quite low. I'm currently around 1900 chess.com tactics rating, decently intermediate I would assume. Would those easier courses be a waste of time for me? I would worry that I may leave a fundamental gap in my tactics vision if I don't drill my head with the most basic and easy puzzles. But would there really be any benefit to drilling myself with 4000 easy tactics or would the harder courses deal with that?

Sorry for the large post, I appreciate any input.