What could be used to mitigate hash collisions?

Key space size applies to symmetric or asymmetric encryption strength, not to hash function collision resistance, as hashing does not use encryption keys.

Appending arbitrary data to a file changes the input but does not address the fundamental mathematical property of the hash function that allows collisions; this describes salting for passwords, not collision mitigation.

Using separate keys for encryption and decryption describes asymmetric (public-key) cryptography and is unrelated to hash collision resistance.

A larger bit length expands the hash output space (e.g., SHA-256 vs SHA-1), which reduces collision probability by an exponential factor because an attacker must search a vastly larger space to find two inputs with the same digest. This is why cryptographic standards have moved from 128-bit and 160-bit hashes toward 256-bit and 512-bit algorithms to mitigate collision attacks.