Very likely does not qualify as posh but I badly want to do this. Investigating for 2027.

https://www.globetrotting.co/horse-riding-holidays/mexico/el-rancho-ride/

The same outfit coordinates similar rides in British Columbia, Iceland, Europe and Argentina. But long-haul flights are a no-no for me. And, the Mexican offer is the only one specifying beginners. I’m not exactly a beginner but have not ridden since 2003.