OP - I think you have heard clearly that being an independent reader with good comprehension by the end of third grade on grade level is the key skill overall to learning. Marg, of course, is important, but it does not impact everything as much as reading. So leave language immersion until the options
until DD has a solid grounding in English.
You also realize that giving DD another year in preschool enables her to gain skills across the board in social, communication. Gross and fine motor etc. all of these impact learning. In public school at least kindergarten is now paced in the reading skill program. There is little time for in school over repetition, so the more solid base you can give her to start iff in K on level will serve her well.
Many years ago we did retail her in two private
Pre-K programs because we found she had the skills to learn to read. Then we were also able
To retain her in first grade as she was able to be I. A regular reading group. This was a long time ago in which she was the first to demonstrate with her disability that one could learn to read. Reading for her also
opened up the ability to learn to play the piano in her teens and it serves her well today as a personal interest and an intellectual challenge.
This will be the only time you can count on to make such a decision, unless perhaps you pull a child from public to go private to do better - tactic often used with immature boys at some point in high school.