Glad you're liking Slow Horses!

Open to recommendations for all 3, but especially saddle up!

Two short things I read and liked, that'd work for Saddle Up: Kiki's Delivery Service by Eiko Kadono (also works for cozy), and River of Teeth by Sarah Gailey (hard mode!). If neither of those sound interesting, maybe look at The Country Under Heaven by Frederic S. Durbin; it's still on my TBR, so I can't recommend it, but it looks like it might be fun? Beyond that, I'd argue that driving/piloting counts for Saddle Up, so anything featuring aviators, professional drivers, or mech pilots should also work.