I've since gone back and watched the first few episodes of the first series and the drop off is absolutely noticeable IMO. It's like a different show.
It's so obvious that since it was the last season with guaranteed huge viewership, Amazon slashed the budgets to maximise value, and it massively suffered as a result. Everything more or less takes place within a series of rooms with a handful of extras. S1-S3 has huge outdoor set pieces all the way through.
And I get that the cancellation of Gen V probably threw a spanner in, but the writing in the last season is so inconsistent. Character arcs that go nowhere. The power scaling was all over the place (seriously Homelander's laser eyes cutting people in half, then at the end they don't even damage their clothes anymore?). The entire V1 storyline turned out to be completely pointless. The dick and arse gags were painful to listen to at times. Essentially Homelander was built up over the entire series, along with the "scorched earth" stuff, and he basically did nothing.
I still enjoyed watching it because I was already invested - but the last season was a massive disappointment with awful writing, carried by the performances of the actors.