Welcome back, Brendan!
I solved this pretty quickly but found a few difficult stumbling points, agree with your difficulty assessment. Enjoyed the puzzle a great deal, thanks to Paul (of whom I'm a long-time fan).
Fairly subtle and cleverly implemented theme. I lucked out and got the theme early, due to a rare clue for which Americans have a solving advantage. 😉
FOI 1d, LOI 1a.
COD: I liked the linked thematics...could pretty much throw a hat over [(2, 5), (17,9), (13,20) and the Pauline 9]...the witty (2,5) is the winner by a narrow margin. A number of other good clues including 21, 25 and 4.