I didn't hate the ending, but as a fan of the book, I think you've inadvertently captured why I can't love these Good Omens "sequels" either.
I enjoy the Crowley and Aziraphale relationship, and I don't mind it being made canon. Tennant and Sheen have great chemistry (as much as I think Tennant is a tit off screen) and I could happily watch a whole show of them playing off each other. The drawback is that this show is supposed to be Good Omens. And Good Omens wasn't Aziraphale and Crowley's story.
People think it was, because they're the POV characters for most of it, and they're on the cover. But if you actually read the book, they're more like side characters who have accidentally stumbled into something much bigger than them, and are just trying to keep afloat. They're not the heroes of the story. The heroes are Adam and Anathema. The point of the book isn't "An angel and a demon fall in love", it's "Human beings matter, actually, and it's wrong to treat them as chess pieces in some great cosmic game".
The show sort of nods at this idea, when Crowley makes his sacrifice for humanity, but it falls a bit flat, because the show itself treats humans as unimportant. The humans don't drive the plot, they don't advocate for themselves - they don't even accidentally complicate anything or save the day.
Compare it with Good Omens the novel, and it's night and day. In the novel, you've got Newt accidentally breaking everything electrical he touches, which ultimately shuts down nuclear armageddon. You've got Anathema and her prophecies - she knows what's coming and she's determined to stop it. You've got Sergeant Shadwell, who accidentally gets Newt where he needs to be, and Agnes Nutter who's pulling people's strings from hundreds of years beyond the grave, and you've got the nun who accidentally switched the babies and gave Adam to the wrong family, resulting in him growing up as a normal child. And Pepper and The Them, who ask Adam which bit of the world would be his after Armageddon, and prompt his crisis of identity. Humans matter in the book. But in this tv finale, we don't.
As I said, it's not that I hated it. But it doesn't feel like Good Omens. The ship just completely and utterly took over, and even the discussion of whether or not it was a good sequel is now all about how the love story ended. And as a fan of the book, I just feel like something was lost in that.