Sounds like you're pretty upset about it, and that's fair. Everyone reacts their own way.
The writing changes massively over the series, because Jim was only 25 or so when he started the series. He just wasn't as good a writer back then. He has obviously also grown and changed as a person over the last 20 years, as people do. And while some core things remain the same, they also refine too.
So reading the earlier series definitely feels different because it's written by a rather different Jim. And that's why sometimes the tone and feel is different. At least that's my take.
Personally, I am rather happy with the changes by and large. Some of Jim's earlier writing was a bit more immature and a bit less well done. But I am not particularly upset about that, as the guy was just doing his best at it at the time. Which is still a lot better than a lot of people out there. And certainly better than anything I have ever written. So you know, I try not to judge too harshly.
I get that there are issues. And I do feel there were some slightly jarring moments. But, he did have a particularly difficult writing period. And to me, he still produced the most interesting and exciting books to date. Sure, they are not the most humorous. But the series has evolved beyond gumshoe wizard goes around Chicago. And that's the nature of long series. If he didn't develop it at all, it would be Poirot or Morse or something. And even they have development, to a point. I think people would give up in droves because their would be not enough exploration and natural expansion and growth.
My advice, if I can be that bold, is just take a beat. Maybe you've just moved on from the series. Maybe you can always reread you're favourites. But I think hanging in for the long haul will be rewarding, if you're prepared to just appreciate it for what it is. Don't lose hope. Jim always delivers eventually imo.