Well the main themes of the Nimzo are control of e4, and quick development and/or a better pawn structure in return for white getting the bishop pair. The most critical lines are the classical (4.Qc2) and the Rubinstein (4.e3) so make sure you prepare for those thoroughly. As I am sure you know the main downside to the Nimzo is that white can avoid it, so you need another opening. When I played the Nimzo I also played the modern benoni. These worked well together because if white played an early Nf3 I would go into a benoni and avoid the critical lines with f4, because the knight has already blocked white's f-pawn.

The QID or the bogo indian defence are more similar to the Nimzo than the benoni is. The downside is that the QID is quite theoretical, and neither of them change the fact that white can still play a Catalan, which means you are essentially preparing 3 openings against d4 (NID, BID or QID, and Catalan) which turns out to be a lot of work. The nice thing about the Benoni is that it avoids the Catalan, which just ends up being a fianchetto benoni I believe.

So yeah... It is important to try out a few of these things you could play with the nimzo. The QGD is another option, but you still have to be ready for the Catalan. Good luck. Edward Dearing's book "Play the Nimzo Indian" is good, although a bit dated (2005 I think), and also a bit too biased towards black IMO.

Ditto to BigTy.

It seems a big jump from Grunfeld and KID (which are different enough from each other) to the Nimzo/QID/QGDO/Catalan complex.  But if you accept there will be a learning curve, it can be worth it, as Black seems to get consistently viable positions in these defense, with White rarely able to obtain more than a "normal" advantage, and retaining some counterchances.

The best way to learn it is to jump in and start playing it, at first in your casual and blitz games, and very soon in tournament play if you compete there.  No point in waiting until you understand everything about the lines, because that day never comes.

I can't add much to BigTy's book suggestions - the only Nimzoindian book I've ever owned is Taimanov's Nimzowitsche-Indische bis Katalanische, published in German in 1972.  But it's been my main defense vs 1 d4 for over thirty years, off and on.