It's interesting you say that because the Discworld books did seem so dark and serious toward the end. Unseen Academicals is one I haven't read (I have zero interest in football which isn't helping) and the last Tiffany Aching book - I did love the Tiffany series so I must at some point read that. Witches Abroad is another fave of mine, the fairytale theme stands out (and I will always love how Nanny Ogg knew how to start spelling banananana but didn't know when to stop!)

I loved the lighter hearted stories and footnotes, and while I didn't mind Making Money too much, all the joy seemed gone in "Snuff" and I suspect you're right that he had less input into the last few. There's other authors doing/done that when they are ailing or just bored with their creations like Wilbur Smith and James Patterson with credited co-authors and it does show in the text.