So my initial instinct is to recommend you make sure to check out some LACs in urban areas with strong Math Departments and that are in suitable states for trans kids. I’m not really qualified to suggest if that only means the deep blue states, or also if you could also consider light blue. The reason that matters is I would be suggesting LACs like Swarthmore outside of Philadelphia, but PA is only light blue. LACs in general, though, can be very interested in kids with strong combinations of academics and interesting life experiences.
But before I get to any of that, the question is if top academicky LACs would count as sufficiently meritorious with your parents. Since one of them is apparently from the US, you might have a shot at it. But you can let us know.
Just as a preview, if you google “College Transitions top PhD feeders”, and go down to Math, the list on the right is per capita (I would link it but the software here sometimes doesn’t like that).
The top 10 in that list includes several LACs: Harvey Mudd at #2, Pomona at #4, Swarthmore at #5, Reed at #7, Carleton at #9, and St Olaf at #10.
The good news is California (Mudd and Pomona), Oregon (Reed), and Minnesota (Carleton and St Olaf) are all deep blue on that map. Mudd and Pomona are both Claremont Colleges so in the LA area, Reed is in Portland, and Carleton and St Olaf share Northfield, MN, which is a (very nice) college town about 45 minutes from Minneapolis-St Paul (which is a very nice large city).
There are more like that we could discuss, and in fact I would have some suggestions of LACs or very nice research universities that sometimes offer merit, which can help reduce the cost difference with, say, European options. St Olaf in fact has a robust merit program. Rochester, a very nice and well-regarded research university, is in New York (deep blue), on that Math PhD per capita feeder list, and has robust merit.
But your parents would need to be willing to think in these directions, so let us know.