I like a flat seat, too. I actually thought the Passier GG had a not-so-flat seat. Mine also had very large knee/thigh blocks. I did not like it at all.
I have a Rembrandt and an old Ruiz Diaz Collegiate. Both are minimalist saddles with relatively flat seats and almost no blocks of any sort. The Rembrandt is cool because it's got an adjustable tree, but I'm not sure the panels are working for its intended horse. The Collegiate is my favorite saddle to ride in after my Rocking R western saddle.
The Stubben Tristan is about as flat as I've seen, and you can get some great deals. If I sell the Rembrandt it will probably be what I try next.
Stubbens have longer tree points than Passiers, so you might need a wider tree in a Tristan. I ended up with an old Spirig, but when I was looking, my Paint, who is wide but not a tank, would have needed a 31.5 or a 32 in a Stubben, but could get by in a 27.5 (M) or 28.5 (MW) in a Passier because of the shorter tree points.
Anyone know how the Tristans, in general, do on a rounder, well sprung horse, like a Friesian?
I did just find a bunch for sale on ebay. (rubs hands together)
I picked up my old Tristan's at Rick's Heritage. One was a custom order from the late 80's and it is a Romanus/Tristan hybrid with an all suede seat & flap and a small pencil knee roll in the flap. It was $150. The other is a standard Tristan and was $500.
They are technically too wide (31.5) for our warmbloods who fit in the new 28/29cm trees but with some shims and a half pad did interim duty until the new saddles came in. I have only ridden one Friesian, so I can't say whether it would be a good fit or not. (sorry)
If you're anywhere local to Rick's, they do have a a pretty good sale/trial policy, so it might be worth trying on a Tristan before buying. Or try contacting a seller on horseclicks and see if you can do a deposit/trial that way. If you're local to us, you are welcome to try what we have here at the farm.
FWIW - Stubben has changed the trees pretty dramatically from what the old saddles (Tristan, Schultheis, Siegfried, etc) and the points are much shorter. Like the previous poster said, it might be worth contacting Stubben or checking out their used inventory - they have lots of wide(r) saddles for very good prices IMO. That's where I picked up our new-to-us jumping saddles and they're in fabulous shape.
Suggest you look for used Passier GG or Nicole. You should be able to find one for under $1500. I have nice one that I would let go for $1200 but it is only a 17" and it sounds like it would be too small for you.