Welp, the saddle search continues, 'cause the saddle I’d hoped would be a dream saddle turned out not to be. 
I arranged with a lovely COTHer to trial a “custom” Custom Advantage with a petite flap. I have a 2006 Steffen’s Advantage (that’s the one with the giant knee rolls that hurt my hips), and fell in love with my trainer’s 2014 Steffen’s monoflap (no hip pain!), so I figured this petite-flap Advantage was meant to be. The gold welt on it even matched my bridle! Destiny, right???
But alas, the saddle did not fit—bounced in the back even with a thick pad. Turns out that my horse has the straightest Morgan back the fitter has ever seen.
She was understandably reluctant to adjust the tree at a significant cost to me, only to have it still not fit. Best we can guess is that the pony panel on it (which my long-backed boy doesn’t need) ended up emphasizing the tree curve, and apparently the pony panel isn’t as thick top to bottom anyway as a regular panel, so it just didn’t work for my boy.
Since I’d trailered three hours to meet with the fitter, I rode in an Everest R and an Omni while I was there. I wasn’t 100% sure of them when sitting, but my boy went much better in both—started swinging his tail and much more willing to round his neck and stretch down. So he has spoken. Other bonus: cantering in the Omni was an “OMG, my butt barely needs to move in this” kind of thing. The fitter thought that might be because the Omni has the straightest tree. I think either a 17 or a 17.5 might work, but my knee felt better in the 17.5 because my knee didn’t have to rest against the raised section below the R1 knee roll.
She mentioned that the Omni has been around for a while, and that might help me find a used one in my price range. I got excited. Unfortunately, at the moment they seem even more scarce on the ground than the Advantage, and I’m thinking I may grow old(er) and die before one with a petite flap shows up. I wish I could throw money at this problem and just buy a new Omni, but I just can’t bring myself to spend that kind of money on a saddle right now, especially one that will take months to get to me anyway.
So, I’m back to thinking about alternatives while I keep searching for an Omni. And now that I know my horse’s back isn’t just straight but super-straight, that narrows my options even more.
I love my Adam Ellis Chloe jump saddle, so I have my eye out for an Adam Ellis Avanti (thanks to @CobJockey for mentioning hers). Anyone know which of the other Adam Ellis models has a straighter tree? Bravo or Style, maybe? What about the Kemlyn? I’m guessing the Brio is more curved, but please correct me if I’m wrong. I emailed AE several days ago but haven’t heard back. Also emailed the “local” (3 hours away) AE rep and haven’t heard from her yet, either. (Saddle searches suck, and so does living in the boonies.)