Well, I don't think he was quite as good in 2018-23 as he was in 2011-16, if that's what you're asking. But even within that, there are rises and falls. And the main difference between those two spans--in terms of Novak's level--might be quantity of events played, and perhaps consistency. I mean, his two best later years (2021, 23) saw him play only 11 and 12 events, respectively, versus the 15-17 he played each year in 2011-16.
Despite its flaws/limitations as a stat, this is also where Elo has value. According to UTS's version, he reached his highest level at the end of 2015 to early 2016, after his best year in 2015 and then winning the Australian Open. See here:
View attachment 10092As you can see, he fell off after Roland Garros in 2016, then plummeted in 2017 into the first half of 2018, when he started rising again in 2018. But he never reached the heights of 2011-16; by the end of 2023, his highest Elo was about the same as his lowest Elo in the 2011-16 span. I think part of that is that he played fewer tournaments, but maybe also because the competition wasn't quite as fierce. I mean, no Federer or elite Murray. So its hard to say whether that lower Elo is mostly due to those factors, or if his level was actually lower.
As you like to say, results have a lot to do with opportunity - especially when you're talking about career totals. With the Big Three, I think they're essentially equal players in terms of their best possible tennis - the margins are super thin - but most of the difference in their career results has to do with context (which includes opportunity). Novak is the best candidate for GOAT not as much because his peak level was higher (which is debatable) but because, in the end, he outlasted the other two and acquired more big trophies. But that's still good enough to consider him, at the very least, first among near equals.