I'm so sorry you had such bad luck but I do think a lot of the problems you faced had to do with the Leghorns I noticed you kept getting them I'm guessing for eggs. Leghorns have a reputation as being a much more domineering breed. I know some people have had good luck with them but I don't think they are very well suited to a small docile flock that doesn't have a lot of space.
I told you about our two brown leghorns. We have one cuckoo marans and she is a bit more dominant but not a bully either. I don't know what was up with that bird. Perhaps it was just a bad egg or went completely nuts. Lol
Anyway that being said I don't think that large fluffy orps would do well in your South California temps they are very high in the summer correct?
We broke 100* here multiple days here last summer with up to 100% humidity and our girls were miserable. They pulled through it like troopers even when I was worried we'd lose a bird but their size and fluff does make it more difficult.
Perhaps something like a Buttercup, campine or something along those lines?
We continued w/ Leghorns because the first White Leg was such a nice humane alpha w/ the Silkies. We were advised and found that as hens mature they get more assertive so with heavier dual purpose or layer breeds they can turn too aggressive around littles. Our White Leg was exceptionally nice up until 3-yrs-old and then she turned aggressive. The Buff Leg was aggressive almost at the start. The Marans was just an out-and-out sneaky hen that avoided human contact except for treats and she would roost next to the Silkies at night to pluck them. During the day she tried to usurp the White Leg's alpha position but got put in her place by the Leg. Then the Marans shifted her attention and outright viciously attacked a Silkie pullet. We had had it w/ all the dual purpose or layer drama queens and started researching docile breeds like Ameraucana, Araucana, Breda, Crevecoeur, Dorking, Easter Egger, Houdan, Pavlovskaya, Polish, Sultan, and some of the reported gentle giants like Bielie's, Brahma, Cochin, Jersey Giant, Orp's, and Sussex. Since Buttercup are from the assertive Mediterranean class like Leghorns we didn't think trying more Med class would be a good idea or trying any of the braekel types like Campine, etc.
We finally chose an Ameraucana as our egg layer and I'd have more of them because of their great temperament and acceptance of new birds into the flock without incidents -- but these sweet overly-fluffy girls suffer in our horrible SoCal summers. Last year the 100o temps started in May and lasted in the upper 90's to over 100 without a break all the way thru Sept. Oct and Nov cooled to upper 80's to 90's. Usually we only get one or two weeks of 100's temps but last year it was solid heatwave for months and we lost our sweet Ameraucana and docile Breda to heat-related issues despite our attempts to keep it comfortable for them. Only the old Silkies made it through our brutal 6-yr State-declared drought. We're expecting this year to be the end of the drought but have no idea if the summer will be milder -- usually our summers are milder after a drought is lifted.
Because the Breda is a lightweight bird we added a new pullet and have another one coming later this year. We're gonna try the Breda's again and hope this year will be kinder to them climate-wise because they are great layers and docile around the Silkies. Wish me luck!