"Rider" - middle grade book series including eventing (equestrian) UK setting

Read probably 12-15 years ago and reminiscent of the 'pulp' style of book clubs that were popular in middle grade. Involved horse riding and I'm pretty sure were eventing based. A series (with a spin-off, not sure if I'm allowed to talk about that here) that I'm pretty sure would be called enemies-to-lovers these days, between rich guy with great horses (money; he maybe owned the yard?) and the groom sorta thing (may be conflating details with the spin-off, I read both concurrently). Absolutely set in the UK, reminiscent of Pullein-Thompson but for a slighter older audience (and definitely NOT written by her).

I read paperbacks in my school library in Australia -- I remember red on the covers (horizontal lines?) with 'RIDER' (possibly Riders?) at the top.

I feel like once upon a time I stumbled upon a wiki page for the series, but extensively googling recently has got me nothing.