I haven't actually finished it (which, weirdly, also goes for the only other shows on your list that I've seen, Gilmore Girls and Marvelous Mrs Maisel - and Mad Men but I've seen too little of that to even really judge), but by all accounts the most recent seasons are a long way off the initial ones in quality... though, of course, they were always going to be judged more strictly.
And this is me as a Diana fan back in the day, but her story lines were just far too terrible in season 6. And they could have been better and more glamorous but it was not to be. Anyway, in the second half of six, one Diana was dead, the show regained its footing and finished strongly. Nothing as strong as seasons 1-4, but still good. Claire Foy was a marvel and its true that British history in the 50s and 60s was much more ripe for television. Still, Olivia Colman's queen against Margaret Thatcher was also amazing. I think part of the issue with seasons 5 and 6 is that Great Britian mattered less and there were fewer stories to work with so they relied more on the rest of the family to the detriment of the story.