"Are you good?" would not be acceptable for this? Just being "ok" would have negative connotations in English. Does beseder also suggest the same?
It can suggest the same, depending on intonation (more stress on the בסדר which imply the negative meaning). But I'm not sure "Are you good?" would fit here. How do you use it?
If someone were to ask me how I am and I replied with "good", it would be a very neutral response. "Fine" would ordinarily be taken pretty negatively. "Are you ok?" would pretty much always suggest that there was something wrong prior to the question. It seems like בסדר is just broader than any one of them.
(Native Hebrewer here) I can't imagine "אתה בסדר?" being asked without a reason to think that something might be not OK - for example, you've just saw someone falling down; or somone told you he's had an accident yesterday. So I think it's pretty much like "Are you OK?"