What city are you moving to (if you know yet)? That may dictate some of the places I would steer you towards. Just off the top of my head, I would say Peacock Hill Equestrian Center in Irvine Park since it’s Central County. There are many English trainer options and it is probably one of the better large facilities for “real” trail access. I can point you to some trainers there if you’re interested.
If you’re going to be in South County, well you’ve got a few options in San Juan Capistrano. Lake Forest has two facilities (including Santiago here even though it’s the Canyon) that both can access Whiting Ranch for trails. Maybe Coto de Caza (Real Housewives was originally filmed here, for reference).
Central/North county- Huntington Beach has a large facility and has limited trail access. Anaheim has a couple boarding facilities with trail access to the bike paths on the riverbed (not gonna lie…these aren’t completely horse friendly and I don’t know if you would find it too inviting). Oh Anaheim Hills facility backs to a solid trail system.
I am in Orange Park Acres. If you don’t want to be with a trainer in a big facility, there are options to board in a backyard here or private barns, Nellie Gail, San Juan, the Canyon areas, some spots still in Yorba Linda and down in the Back Bay in Newport Beach. Some of these areas still have community arenas and trail access. Just depends on what kind of trail you like riding.
Equestrian access and facilities are really limited in OC. And turn out is almost non-existent. Mostly pipe stalls and dry lots on very limited acreage. As long as you are prepared for that, you should be ok.
Feel free to PM me if you have any questions. I’ve lived here with horses my whole life. Hopefully the collective SoCal COTH community can point you in the right direction.