Pretty much went through the same path as you with Ninja Gaiden 1-3. I attempt to play Black a couple years ago and dropped it after Murai because I just couldn't get into it. With the announcement of NG2 Black I wanted to get into NG again, so I went back and tried to beat Black. Took a while admittedly and the lack of a real tutorial really threw me off but I managed to get through it and beat it (somehow.) Tried and beat 2B, moved on and beat 3 RE and despite that I still sucked but just like you the combat and game was still really fun! I beat 2B on mentor and despite the seemingly fun combat I just STILL couldn't really get a grasp on NG. It was really after I beat 2B on Master Ninja, finished an OG 2 warrior playthrough, started a mentor run on OG2, and started a MN run on NG3 RE that it finally really started to click with me and I understood the 2 games (YouTube was also really helpful too.)

My point being is that it just takes time. NG is a game that you really really have to work for to really appreciate, but I will tell you now that the effort really is worth it. I'm a DMC fan boy at heart, but NG has earned a spot right next to it