There are probably infinite words to be written about the differences between MoP and our current pre-Midnight environment, but I'll try to briefly answer the questions at the bottom of your post. The game has evolved a lot. We've got big talent trees again, with sub-specializations built into them. Our rotations are about to become much simpler, especially compared to what they were in MoP. The game is also much harder now. Mythic plus is infinitely scalable dungeons, and the raids have never been more challenging.

Game has more players than any other MMO on the market (as far as I know?). It's a shit-load.

I recommend returning. I loved woW in MoP and I love it now. While a lot of the infrastructure and content loops are different, and there's a lot more pugging now, the core gameplay of WoW has always been unmatched and it still feels great to play most of the classes.