Apparently there are quite a few experts in here who actively watch Leverkusen matches
Havertz' 19/20 is definitely worse than 18/19 but scoring stats are the worst way to define that. His hype last season primarily stemmed from his scorers so it doesn't really come as a surprise that he's now downplayed because they don't look as good. Thing is, despite his great stats, Havertz isn't about goal scoring or assisting like someone like Michael Ballack (who he's compared to quite often, wrongly IMO). He's extremely good at finding solutions in close spaces, has exceptional ball control and a really great first touch.
He was out of form throughout large proportions of this season. Not because he didn't score but because his overall game was pretty meh. Part of the reason for that is that Brandt left and nobody stepped up to fill that space. Havertz had nobody to combine and so our opponents could focus on him completely. You don't see that in the statistics but he's getting man marked in every single game. Often he's immediately surrounded by two or three players when he gets on the ball. Of course that won't be the case when he's playing for a better team with better individuals than us.
I also think he's nothing like Kaka. Kaka was a very direct player who was also great at longshots. Havertz is a completely different player. He rarely has good shots from outside the box. He's primarily a midfielder whose very good on the ball, exactly considering his height, but not a direct one. He likes to play short passes, combine out of pressing situations, etc., a bit like de Jong exemplarily. However, he's got exceptional game intelligence. His runs are great and his timing when he gets into the box is phenomenal. And he's got the composure and technique to be an excellent finisher. I agree that his passing isn't on Özil's level but IMO Mesut was the best of his generation in that regard. Havertz still plays great through passes, shifts the play brillantly from one wing to the other and is a very good short passer.
So what you get is essentially a Barca midfielder who's slightly less agile than Iniesta, de Jong and co. because of his height. To compensate that, he's extremely dangerous in the opponents box and will probably get you 20+ scorers a season quite constantly in a top team.