I'm a lot older, but I attended an International Boarding School as well for middle and high school. I also had a similar sense of disillusion when I came to the US. I have a BS in STEM (and I took a lot of liberal arts classes) and a PhD in a non STEM field. Throwaway account because I value my privacy.
First of all, look up the term "Third Culture Kids." There's a book about this topic by David C. Pollock and Ruth E. Van Reken. I read that book years after becoming a proper adult and there was a lot in there that gave me some perspective on how I felt like I didn’t fit in. I think it would have been useful when I was an undergrad.
I’m going to now speak from the heart based on my experience. Please don’t take any of the wrong way, and some of it may not apply to you. This is a very personal topic, so some of it may be very strong.
International Boarding Schools are an artificial environment where kids live very privileged lives. The majority (not all) of the students come from privileged backgrounds. Many of those students are there because their parents want to give their kids a stability they cannot offer at home due to jobs (e.g. parent works in a job where they have to move countries every 2 years), country stability, (e.g. parent(s) who earn well, but live in country that is politically/economically unstable and/or has a bad education system), or the parents migrated away from their home country and they want their kids experience their home country. There are plenty of other reasons, such as abusive or neglectful parents who just need to keep their kids busy somewhere till they are old enough to move out. Not everyone is economically privileged - sometimes the schools will give discounted rates to missionary children from affiliated religious groups.
This creates a highly artificial environment with a particular kind of diversity. At least in my school, there was plenty of diversity of where people were from, but limited diversity of economic backgrounds or financial status. Barely any students were poor or lower-middle class. Nobody had parents who were blue-collar or did manual labor jobs. We could all celebrate Easter, Eid, and Diwali because we were far from places where being a certain shade of skin color or religion disadvantaged you for life.
In these schools privileged, culturally diverse, kids can be ambitious because they have a strong safety net, where you can have diverse conversations about poverty and colonialism in Africa while ignoring all the school workers (e.g. kitchen staff, maids, custodial staff) who live in poverty while making sure you have food to eat and clean your toilets. And you can take classes like “Theory of Knowledge,” which teaches you some things, but also makes students arrogant about how smart they are.
If you are looking for a place like that, the only other place like that is another international boarding school.
The first thing to realize is that part of what you are missing is having a common background with your classmates. They are talking about things that are relevant to them based on their shared experience growing up in the US (I assume you’re in the US?). You have a different experience growing up.
The topics they want to discuss may seem like a “regional microcosm” to you, but the topics people talk about are based on how they grew up. Even the TV shows they watched as kids might be totally different from what you did.
When I was an undergrad, I knew some American kids would get frustrated with International Students because they felt like what we spoke about was rooted in our past experiences. Did they also feel like International students only spoke about things that were our “regional microcosm” and we weren’t open to the American experience? Did they feel we weren’t being diverse?
I’ve been in the US for 20+ years, and not having this shared background has been a hindrance to making friendships, even today.
Beyond this, you probably also now interact with people with a diversity of economic backgrounds and social status. My first year of undergrad, there were people on my dorm floor with parents ranging from PhDs to high-school dropouts. It was really difficult connecting with some people. By my 2nd year, over 50% on my floor had dropped out, and the majority of those had parents in blue-collar jobs. By my 3rd year, even more people had dropped out. It was a lot easier to talk to people since a bigger percentage of the remaining students had safety nets and the mental space to sit around and talk about a wider range of topics, and not how their parents were pressuring them to join the family mechanic business instead of studying mechanical engineering.
I know more than a few international undergrad students who were able to sail through the first year, get crushed towards the end of the second year as things got harder and they became lazy, and then have to relearn to learn in their 3rd year.
Personally, I wouldn’t make the assumption that you’ll find the community of like-minded people in liberal arts. As already said, a lot of them won’t have the experience you had in boarding school. Also most people who go to college do so because they have to. Education and Liberal arts (psychology seems like a big one) is a common place where people land when they don’t know what to do. I say this having been a student and an instructor (during my PhD years) of social science classes. Most of the students are disengaged. Some are only temporarily passionate about whatever new topic allows them to quietly boast about their “critical thinking skills” (e.g. learning about cultural relativism in anthropology). You’d be lucky to find 3 students in a class of 20 who have a sustaining passion about anything they are learning.
Finally, a comment on the long term view about leaving a STEM major for a liberal arts school. At least in my boarding school, most of the people were competitive from a career standpoint. Everybody wanted to become a lawyer, doctor, business owner, executive, etc. This created a lot of internal pressure for people to have jobs of a certain status. And about 5-6 years after high school was over, a lot of the “lifetime” friendships broke as the job status gaps appeared. And some people (including me) went through rough personal periods in because we couldn’t achieve what we saw our boarding school classmates were achieving.
International Boarding school or not, it crushes your soul when you finish your degree and you can't get a decent paying job for years. Unless you are the type of person who has zero issue enjoying the money your parent’s earned, that will slowly eat up your insides. A STEM degree is some degree of insurance against that.
Before you make any decisions, I'd encourage you to reflect on what you feel you are are missing and if you've correctly understood where you can find the fix. I attended two schools and I found the bigger state university attracted a more diverse set of people and did a better job retaining students, which gave me a more diverse environment.
All of the above is a reflection of my experience and might not represent you. I apologize if I’ve offended you or said something you feel is just an assumption on my part and mischaracterizes your experience.