How long would it take to crack a double hash if salt and final hash are public?
A SHA3-512 hash outputs 512 bits. If you use that as input to the second application of the hash then it takes $2^{511}$ tries - on average - to brute force it. This is obviously far out of range of a practical attack.

The one thing that could be performed is to try to guess the input of the first hash: the file content. If this can be guessed then the scheme would be broken. The file could be very short or it may just not contain enough entropy, regardless of the size. For instance, if you'd use the copy of the MP4 version of Kill Bill 1 and somebody knows that you like the movie, the hash may be relatively easy to guess.