Looking at this list (and without walking a few yards to the bookcase), I started on hardbacks about book 18.
No backfilling, earlier ones read as paperbacks.
There's actually a - tenuous - motorcycle connection.
In about 1994 I was going to the Birmingham bike show with my mate Paul. We stopped at a services and he was looking for the latest Pratchett. I'd never heard of him. Next day (we did a day on the BMF stand (RIP) and a day wandering) Paul arrived with Colour of Magic and Light Fantastic. Hooked. Filly bought me Masquerade in HB.
https://www.discworldemporium.com/reading-order/