Robert Rankin's Armageddon trilogy is a good start - plus unlike the Brentford trilogy or the Hitchhikers trilogy, it's actually 3 books
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. Pratchett was a big fan of that series by all accounts. Rankin does get a bit samey after a while, and you do need to read them roughly in order - there's a lot of running jokes between books, or surprise pseudo-sequels
Tom Holt would be worth a try - much like Pratchett, I found the earlier stuff better - Flying Dutch is a standout
Christopher Moore maybe? I read a few of his early books (practical demonkeeping was brilliant), but I can't comment on much past that.
With the caveat it's been 25 odd years since I read them, I remember enjoying Simon Hawke's Reluctant Sorceror series - American author, but a real Pratchett vibe to them
And although not discworld type books, the Red Dwarf books are really brilliant - much cleverer version of the TV show, but with all the same humour beats. Notably there are 2 book 3s - one by Grant, one by Naylor - very different books, but both equally good.