I’m sorry about your trees. Are your trees in ground? Did you over amend with organic matter? What type of fertilizer did you use?
The symptoms do look like Phomopsis but the only way to tell for sure is to send out sample which most of us aren’t willing to do. Since you are in a desperate situation, I would recommend using Abound after you familiarize yourself with the instructions and warnings. The rate would be 1tsp/gallon of water. Class 11 Fungicides are no joke and if used improperly, organisms will build up resistance. Maybe someone in your area has Abound or another fungicide that is effective against Phomopsis. If you can get Abound and decide to use it, read all instructions and warnings carefully. It can kill Apple trees for example, even in minute quantities. You also want to make sure you wear appropriate Personal Protective equipment/gear.
If you catch symptoms early enough and immediately spray, you can save trees. Your trees are close to the tipping point of no return but there’s still hope.
Many Florida trees planted in SoCal end up like this and I’m glad you were brave enough to post this and ask for help. Many other people have experienced this, myself included. Usually the Florida trees take several years to look as bad as yours so I’m guessing there’s something about your yard/climate/diseases in your yard or plant husbandry that may have accelerated your trees decline.
Hopefully you can save your trees but if they don’t make it, try planting Mango seeds of Polyembryonic and Monoembryonic varieties and graft onto the most vigorous seedlings.
I love Mangos so much and I hate seeing people loose trees like this. This is one of the reasons why I perform all my experiments. I may be selling some grafted trees on seedling rootstocks in the coming years.
Simon