> it felt like there was more and more philosophising

If you're referring to the much-later books like Unseen Academicals, then sadly I think Terry's alzheimers affliction was probably to blame. As I understand it he reached a point where he was no longer able to type, but could still dictate. Certainly somewhere in that process they lost the tautness of the earlier works. Something to enjoy still (for me) but not quite the glittering jewels of his intellectual peak.