"But the set of words that is introduced in early phonics uses only the easiest spellings.
So yes, given there are a lot of sound-to-letter correspondences in English, it is hard to become a fluent, read-anything reader in English.
But it's no harder to learn to read r-a-t in English than r-o-s-a in Spanish, if you are first taught the easiest 20-something sound to letter correspondences."
Sure. But the thing is, because there is so much more to learn AFTER the r-a-t type words, there is a need to start at an earlier point.
It takes about 2-3 years to get really fluent at reading English, so you want to start phonics instruction at 4-5 if you want them to be able to read their textbooks and worksheets and what teacher has written on the board easily by age 7 or so, so that they can get cracking with learning other subjects like history and science.
In other countries with easier writing systems, kids can cover the mechanics of reading much more quickly, so if you start at 6 or even 7, they will also be at the "being able to read a children's textbook" stage by age 7.
By the way, it's not a binary choice between "PANIC!!! WRITE HER OFF AT AGE 5" vs "She'll pick it up when she is ready." Of course the OP should not start writing her kid off at age 5... or panicking. What she should do is do some extra work with phonics and blending, to help her daughter get over her current area of weakness.
I don't think "She'll do it when she's ready" is helpful at all. She might just suddenly pick it up, she might not. Loads of older kids have serious reading difficulties so these things do NOT necessarily just resolve themselves.
And reading is not a "natural" skill---"readiness" is a nonsense concept when it comes to reading because our brains never evolved to be able to read in the first place, so they can't be "ready" for it at any age.