A lot of men like this just cast a wide net, pay every female they are attracted to compliments and attention and see where it leads them.
I've seen men in clubs do this. They literally just go around the room approaching every female until one responds.
He waited until you were 16 as he didn't want to break the law.
(Legal is different from Morality., BTW. I still think he's a shit.)
Obviously you're right to be upset about this but maybe it would help if you separated out the elements of why you're upset.
I totally agree that he took advantage of a significantly younger girl (turns out he wasn't like 21) who was at a low ebb, and, yes, he sounds like a nasty piece of work.
No argument about that! That, to me, is an absolute given.
You use the word grooming, though, which is a very specific term.
Is the fact that you think he sort of had some sort of a masterplan particularly upsetting because as I said in my first sentence, I'm not sure that he did.
Again I only say this as you used the word grooming, I mean if you said was I taken advantage of by a predatory older man who turned out to he a piece of shit when I was at a low ebb? the answer would just be, absolutely you were.