Jenku Dietrichsen (https://dressageadventures.com/) seems to include at least some R+ in his training. He uses a marker signal and food rewards. I’ve gone through most of his “Adventures with Jenku” content and he does incorporate some R+ and certainly plenty of standard R-. I think what he does in his “Scatterproof” course has some strong parallels to CAT/CAT-H constructional approaches, if you are familiar. I believe he’s helped train some fairly competitive riders, and is based out of S. Africa. I did appreciate that his horses seemed generally happy, calm, confident, not over-threshold during training.
Shawna Karrasch and Jessie Hillegas are very interesting. I am part of their membership and think they are pulling in the right general direction. They are very R+ and Shawna brings the science, while Jessie is a H/J trainer that - interestingly - is a convert to R+ from a pretty traditional, successful H/J background. I think their base right now is mostly beginners and folks that are new to R+, but I hope to see them really grow over time.
Clicker Expo actually has some horse components now! When I attended virtual Clicker Expo a couple years back, I signed up as a mentee in their mentorship program and was paired with Mary Kay Hasseman, who I believe incorporates R+ into dressage work. I think you(g) can get a lot out of Clicker Expo and other really cutting edge R+ sources (regardless of species) IF you are pretty self-motivated and a little nerdy. I would also recommend having a look at FDSA (https://www.fenzidogsportsacademy.com/) - yes, it is largely dog-centered, but I recently took a great class with my horse and got a lot out of it. There are some classes offered that are open to multiple species, including horses.
Monty Gwynn, Alexandra Kurland, and Michaela Hempen are, of course, some of the names in R+ horse-dom that I can think of. I have read Kurland’s recent book… I have to say, her writing style/presentation-style is REALLY not my cuppa tea… but she does have some nuggets in there. Her understanding of errorless learning is excellent (…even if it does get mushed in with a bunch of fluff in her book…).
I guess - slight tangent - I would reeeeally love to see more horse people get into the science of behavior far enough to understand quadrants, operant vs. classical conditioning, CER, markers, etc. Some of those basics that explain why a certain training approach may (or may not) work. R+ with horses is remarkably fun, and I’ve been pleasantly surprised that nearly all of the myths I’ve heard (and believed!) about food and horses are totally untrue.