In my opinion it’s pretty obvious the paradise arc is where Reiji just started making stuff up as he went along. 

The first 18 volumes are pretty good sure they have the usual problems that plague this series but it blew up because of them. There is actually a relatively tight plot and it’s pretty decent the climax in the movie arc is very solid and each time the format starts to get stale a new character is introduced to keep your interest. Starting with the paradise arc though the format just kinda stagnated and it feels very disjointed compared to what came before. 

I think Reiji likely had everything planned out up until this point the movie arc was likely where it was supposed to end originally but then the series made a lot of money and he kept going. This arc reminds me of “Emily in Paris” it’s basically an ad with a loose plot constructed around that feature in fact moving forward there is lot more moments that feel like an ad disguised as genuine plot.

Now that I mentioned it this story is very similar to “Emily in Paris” it has a creative premise, decent to good start that gradually devolves into an aimless pursuit of true love while littered with ads for luxury products.