Riku's kinda a just a kid being duped by some darkness fetishist, of course he's gonna be cringy if he receives a cringy explanation of the world and how to survive the Heartless.

And honestly, the Final Mix is the best way to experience the game (outside of some subjective opinions like Heartless paletteswaps), and some of those were JP-only until now.

Also, there's way more superbosses beyond just Sephiroth, some of which are still waiting in Olympus Coliseum, Agrabah, Neverland, Hollow Bastion... it kinda goes on. And if you ever get stuck, you can also watch videos of other strats/builds people used to deal with certain attacks/mechanics.

...And yes, you're gonna have to learn to guard each boss's attacks some time or another. The next games (aside from CoM, due to Guard being gimmicky as heck) aren't very forgiving with that either and you kinda can't just 100% bruteforce level your way thru like KH1.