I'm curious why you think there was ever a steep decline in quality, the popular opinion seems to be that the series has stayed consistent throughout or even improved. I think you should watch season 12 at least, it's unequivocally the best of the newer seasons, just classic episode after classic episode.
I'm 7 episodes into season 13 and so far it's wildly inconsistent, basically a 50/50 split between "the best episodes of Always Sunny I've ever seen" and "the worst episodes of Always Sunny I've ever seen".