Based on my limited knowledge and understanding, I think you should first narrow board on the board/curriculum. And from what you have described, your child might be more suited for IB/IGCSE. Big or small, schools with international boards typically have not more than 30 kids in a class.
I agree, ICSE might be a better option. Focus is more on getting the basics right until they reach a certain stage. You may want to consider future kids. Their approach is little different than international schools, ICSE and their principals are very friendly folks, they understand your concerns better. But remember ICSE gets tougher as you go to middle & high school, their syllabus is vast compared to other boards.