Looking at your yard I suggest you stick to commercial feed. Your ISAs may struggle with a lack of metheonine. It's very hard to provide in a natural form at an acceptable price.I have 2 high production hens (ISA Browns) who are 4 years old and then I have all other heritage breeds who lay large eggs but not as big as my ISA Browns lay.
The only vegetation they have access to in the summer is grass, sprouts from their spilled feed, strawberry bushes, and a large raspberry bush. They also have access to a compost that they can dig and forage in.
Here are some pictures of their run (in the winter):
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