I have a Wintec dressage saddle for my clyde mix girl, I had to order the wide size gullet since it came with just hte medium/narrow. Fits her perfect, she even has a really low 'mutton'ish wither and it stays put well. I love that saddle and it was only 300 bux, used. Since your horse isn't a full Perch you shouldn't need a specially made saddle.
A friend of mine has wide horses. Her Shire x Paint (more Shire built than Paint tbh) and MFT are fat and round/flat backed with no withers, requiring an extra wide saddle. She rides in an Isabelle and a synthetic Wintec dressage saddles, both with extra wide gullets. Both saddles fit the ShireX with no problem at all.
Some Wintecs have interchangeable gullets too, so if you can find one (nicely) used in a 16 or 16 1/2 seat, you should be fine. I find the Wintec dressage saddles to be really comfortable compared to many of the jumping and A/P saddles I have ridden in, so I have no problem doing trail riding, arena work, and jumping in the dressage saddles with my small build. I ride in a 17-18 inch saddle just for the sake of my long leg. I'd probably fit in a 16 inch otherwise.
He's a super cool guy! He does lower level eventing.
Rocko wears a Bates saddle, with one of the wide gullets. He's not mine, so I can't tell you which it is, but I do believe that it's one in the "wide" kit, with the purple gullets? You'd have to measure your horse individually, obviously, but saddles like the one you're looking for do exist!
I think it's this one, or maybe this one, or possibly this one. I don't know for sure, but the saddle fits him really well and it is SUPER comfortable to ride in!