I still have one of those plain flap Collegiates as well! They were wonderful saddles while they were still being made by Ruiz Diaz for Miller’s.
You could possibly look into a used older Collegiate like the one you have, as I sometimes see them on the market. Mine (from 1996) seems to still be a pretty versatile fit on quite a few different horses. From my understanding, when Miller’s went under around 2002, many of the former employees (along with Eisers) went on to form English Riding Supply (ERS), and a lot of the manufacturers went with them. So, while the Collegiate name went to WeatherBeeta, they changed manufacturers and the saddles aren’t the same, but the Ovation saddles by ERS (and I believe the Pessoas as well) are being made by Ruiz Diaz, who made the original Collegiates. The styles are definitely updated and you won’t find a plain flap version, but there may be more minimalist styles available. The early-mid 2000s Ovation models are identical to the Miller’s era, pre WB-acquisition late '90s/early 2000s Collegiates, though most of them have padded flaps (but not very big ones, and only pencil rolls underneath IIRC - definitely not like the very padded saddles with huge blocks). Ruiz Diaz also made saddles for a few other brands at the time, such as Circuit (Dover), Thornhill, and a few others that are escaping me at the moment. The Circuits changed a lot over the years and seemed to switch manufacturers quite a bit, so you’d want to look for an RD stamp on the sweatflap.
Perhaps a broken in Crosby would work as well? I also have a Crosby Sovereign from 1999 that is very similar to my Collegiate, just made with smooth bridle leather. While it has low miles and is barely broken in, I don’t find the leather to be any stiffer than the Collegiate. IIRC, a lot of the identical designs shared the same trees as well.
As for more modern options, I’ve heard a lot of lovers of minimalist saddles say they like the Butets and Luc Childerics. I haven’t personally ridden in them, so I’m not sure how they compare, however. I’ve seen a few older plain flap Butets and occasionally a Devoucoux on the used market as well. I’ve also heard people say that the newer Circuits from Dover are pretty similar to the older Butets, and they’re at a much lower price point (I’ve seen them even lower used). I don’t have personal experience with those, either, however.
I’ve seen a few Stackhouse saddles that are still pretty old school in design (even a few plain flaps), and I think I’ve seen them still available new from Bennett’s Hunter Fine Bridles (Jimmy’s son) at one point, though I’m not sure if they’re still available. Some of the older Counties such as the Pro-Fit and older Stabilizers might be good choices as well.