Yep the end of FFX is pretty easy, especially if you bother to get ultimate weapons or level up a bit.
Did you also fight all of the monster arena creations because they are supposed to be hard, so there's an extra challenge there which is worth getting strong for. Also I don't know if you have the PAL version or not, but it has Dark Aeons which are (imo) a BIG challenge.

I think these are supposed to be what you need to max out your stats or level up a LOT for.

But if you have the version without dark aeons, and have killed the monster arena creations then that's the whole challenge.

I agree that (aside from the dark aeons and monster arena creations) Yunalesca was probably the last compulsory challenge, although even she wasn't terribly difficult....

And having auto-life at the end is weird, definately makes it too easy.

However I do have the PAL version and haven't killed the dark aeons yet so FFX still holds a lot of challenges for me if I play it again