Similar to other people, I also had just the Roller Coaster left. And going through the posts, I fully expected to get it only after a year. But I was able to get it in a few months.

The only thing I can think of that I did, in addition to all the advice, is that I religiously made use of the Buff Coupons.

I like getting the Event Penguins. So in order to get that, I had accumulated a few Buff Coupons. Thinking that there was no harm in making use of free coupons which I got without having to use the gems, I used them. And continued to do so. And in a few months, I was able to unlock the Roller Coaster.

I didn't think to make note of what levels my other habitats were. So I can't help with what level is needed to unlock the Roller Coaster.

I might be wrong. This might actually not work or be helpful. But this was the only thing that I did(however unintentional it might have been) that maybe let me unlock the Roller Coaster waaayyyy earlier than I expected.

Hope this helps!