^ For most of those schools, it’s easier to get in as a freshman than transfer but not all.
However, as @MYOS1634 said, there’s a big difference in selectivity between the most elite private unis and LACs (almost all of whom range from small to tiny by UK standards and are tough to get in to as they just don’t have many slots or students leaving) and some much bigger less selective ones (many of whom take in a decent number of transfers). What do you mean by “top 50”? The top 50 Research U’s + the top 50 LACs as ranked by USNews? Something else?
BTW, if you have already taken a gap year, Columbia GS is a possibility, though you may still need a compelling story.
Also, I would say that odds at BU and (especially) NEU are a lot higher than 10% (NEU take in International full-pay transfers who barely can speak English as they provide money but don’t impact NEU’s rankings as the stats US News look at are based only on fall-entering freshmen).
“I doubt those admission officers would take “i don’t like what i’m studying and want to try out something else” for an answer.”
That would actually be one of the more compelling reasons for a transfer.