I don’t know much about the colleges at U of T. Hopefully someone else can answer.
Regarding admissions, Ontario has some sort of common application (OUAC 105) for the many schools in the province which I am not familiar with. An unweighted 4.0 should be sufficient at any university in Canada, and Toronto is slightly easier to get into than McGill (and possibly Queens) just because it is so large. 30 ACT is probably enough (one daughter got into McGill with 1410 SAT (out of 1600), which I think is pretty close to equivalent). I am not sure whether Toronto will require an essay but you probably will be fine either way. Yes, your GPA makes you very competitive for admission to any university in Canada.
I do know someone who switched majors during her first year at a different Canadian university. Waterloo when I checked (a few years back) didn’t even let you declare a major until after your first year. I would be very surprised if you had any trouble changing major at least through the end of the first year. Of course, if you change major then you change what courses you need for graduation, and you might need to catch up depending upon what you switch from and what you switch to.
I know someone who did their bachelor’s at Toronto and their master’s at Stanford. I used to know someone else who did their bachelor’s at Toronto and their master’s and PhD at Princeton. It appears that the top universities in the US do indeed know how strong Toronto is.
U of T is an academically very demanding university. It has been said that the top US schools are harder to into and easier to graduate from. The top Canadian schools are easier to get into and harder to graduate from. Regardless, I do know a few people who seem to be able to pull off very high GPAs at Canadian schools. They generally need to work very hard to do so. U of T does have a strong reputation. My biggest concern if you are premed is that it is very difficult to pull off a high GPA at U of T.
We have found admissions to be very helpful at a range of schools in Canada, and I did at one point talk to admissions at U of T and I found them very helpful also. You should feel free to ask them questions. Email should work fine.
There are multiple other very good schools in Canada. You might want to check the Maclean’s magazine university rankings. Universities in Ontario may be on average a bit more expensive for international students than most other Canadian universities (I am assuming that you are not a Canadian citizen nor Canadian permanent resident), but less than the sticker price at most US schools other than in-state publics.