I’ve been a Ninjago fan since the marketing first came out in 2010, and it has definitely declined. I’d say the first crumblings were during Skybound in the first half of 2016. Now I liked Skybound, but I had the distinct feeling upon finishing it that it was a side story, a well-written addendum to the main plot, which in my view more or less wrapped up at the end of Possession/2015 (coincidentally–or maybe not–coinciding with the departure of the original Garmadon character from the series).
Since then it’s been an accelerated train wreck. Day of the Departed was abysmal, Hands of Time only slightly better (though they took a stab at freshening Lloyd as a character by having an ultimately pointless “becoming a sensei/master” arc). The redesign of 2018 was where it really took a turn for the worst. I know a lot of people like Sons of Garmadon and Hunted, but I found them cripplingly disjointed from old Ninjago and pretty empty overall (Hunted especially). The writing of SoG was, admittedly, better than HoT.
March of the Oni was a joke, in no small part due to its titular villains, who were reduced, like the Skrall in The Legend Reborn, to a bunch of grunting primitives bent on antagonizing the ninja for… reasons. Then came the new-new-new reboot in second half 2019, and Ninjago took its last nosedive. “Seasonal rot,” as you put it, has been chronic for a long time but never as apparent as in Season 11, which was just a bunch of filler with two-dimensional villains, cringe-worthy writing, and no ramifications or consequences whatsoever. I almost haven’t bothered to fully watch a season in English ever since, because Ninjago has become a mere “villain of the week” or really “theme of the season” circus act. You had “ninja in a videogame” for Prime Empire, then “ninja underground” for Master of the Mountain, then “ninja on an island,” “ninja underwater,” etc., etc. It got really old really fast. There are no longer tangible connections between seasons or noticeable growth among the characters. Everything has stagnated storywise. The plot, when there is one, works toward nothing higher than a brief and often disappointing climax that’s quickly forgotten in the ensuing shenanigans of the next season’s opener.
I will give Seabound a little bit of credit, because it’s the only season I’ve watched fully in my native language since Season 11, and I enjoyed small aspects of it while watching. I still find it forgettable, though, and it really does nothing exceptionally new. Not even the ending is that impactful considering how many times we’ve gone through “lose a ninja, gain a ninja back!”
Set-wise, I think Possession was also the high point. I’ve found many of the proceeding waves lackluster. There are some fantastic sets, like the Legacy Storm Fighter, and some decent waves–Season 11’s sets weren’t actually too terrible considering their subject matter. Overall, though, the sets have taken on the rehash-y feeling of the show in addition to being generally dinky and oddly conflicting in season-to-season aesthetic (not that there shouldn’t be differentiable aesthetics for seasons, but there’s a big difference between the cohesiveness of the first five waves and, say, the current scuba-fest).
I’d say that’s my two cents, but that’s more like twenty dollars. There you go. 
