I'll add some photos tomorrow of the hillside.

Lay down a foot of mulch and it will be ready for next year.

The dump for the city of San Diego it is located off convoy on hiway 52.

Years ago, we had a hard-pack adobe "garden" in Vista that we did transform bit by bit. First we added a bunch of perlite that we got free from a place in Escondido because it was a by-product of whatever they were making - acoustic ceiling stuff? I don't think they're there anymore, but you can buy it in bags from garden stores. Another great freebie is the used mushroom culture from Mountain Meadows Mushrooms in Escondido. This is first come, first served and people bring big trucks to haul it away, but it's great stuff. They have a web page. Anybody around you have chicken or rabbit operations? That's good. If the dump on Oceanside Blvd (El Corazon) is closer than the one on 52, they have compost, but I wouldn't use it for food crops because who knows what's in it. If you happen to live in Oceanside, the compost from El Corazon is free, otherwise there's a fee.

The good news is that clay is actually very fertile and lots of crops will do fine even when it's pretty hard - squash, tomatoes, green beans, corn, that sort of thing. Carrots, potatoes, not so much. Beets did OK.

Before you go adding fertilizers like rabbit manure willy nilly, get the soil tested to make sure it isn't already overloaded with potassium and phosphate.

UMASS is a pretty good place to send soil samples.

https://ag.umass.edu/services/soil-plant-nutrient-testing-laboratory