Even without the keyfile, a 64 character password alone would be effectively impossible to crack, even by the most powerful computers known to man kind. That said, that doesn’t mean not to use a key file. Keyfiles can protect against attacks like keyloggers which could record your passwords or malware that only exfiltrates passwords and the likes.
As a side note: obviously do not store your keyfile with the volume itself. You could store it locally on a drive, hide it (like using a random image/text file) in a different cloud storage.
You do not need to split your volume, it obviously would make it harder for your adversary to access, but even the NSA using all resources against a 64 character password and a keyfile would struggle with that alone. You also shouldn’t worry about quantum computers either:
Grover’s algorithm gives a quadratic speed-up against brute-force on symmetric keys: it turns an n-bit key into an effective n/2-bit key. AES-256 (256-bit) thus offers ~128-bit security even against a quantum adversary—far beyond any practical reach today or in the near future.
Header keys are stretched via PBKDF2 (SHA-512) with hundreds of thousands of iterations. A quantum adversary still must run Grover’s search per hash iteration, so your high iteration count multiplies their work just like it does classically.
You have covered your bases well. This is, for all intents and purposes, unbreakable.