Sorry for delay replying, just saw notifications on this thread!
Re schools - there are plenty that have a good mix of foreigners and locals. Many will claim around 50/50 but that typically counts all the mixed Thai/foreign kids as foreign, when most are more culturally Thai (though that is a generalization).
Patana is definitely considered 'top 3' and actively restricts the numbers of Thai students in order to maintain diversity. I live nearby and have many teacher friends there. Yes, it is elite and there is a lot of money there. But the teacher kids get on fine, and they are not wealthy. Be prepared to pay a lot extra for all the amazing ECAs (extra curricular clubs). The school bus is also super expensive. The campus is amazing.
St Andrews Nord Anglia does have a fair mix, as far as I'm aware. Also great SEN provision.
@Anatomical I taught at 107 2018-2021, perhaps we know each other???? I agree International Day was awesome there, my fave day of the year! My kids were very happy there too.
Other schools to consider with a good/reasonable mix of foreigners among the locals include NIST, ISB, Bangkok Prep. Ones to avoid if you don't want your child to be in a tiny minority of foreigners are Shrewsbury, and probably Kings and Wellington (they may become more diverse but they're very new so still growing their student base).
I agree that older kids can and do go out and about safely here. My neighbours with kids from early teens up allow them to take the BTS (skytrain) alone to meet friends at malls for cinema/bowling/ bounce/hanging out etc.
I also agree that there is some natural division socially, with kids of different first languages tending to band together. Having said that, my middle child has a really diverse friendship group with 2 Thais, one Chinese American, one Indonesian American, and one v mixed kid (both parents third culture kids of different backgrounds). It does depend on each class/year group - even in a pretty mixed school, some classes will happen to have more locals and others more foreigners.
If you have any more questions, feel free to send me a PM, happy to help!