How secure is Shamir's Secret Sharing for password sharing when attacker has t-1 shares?
Shamir secret sharing is information theoretically secure. This holds as long as the attacker knows fewer than $t$ shares. So, even with $t-1$ shares, the attacker still knows nothing about the secret. So a brute force attack given $t-1$ shares is impossible, even an attacker with infinite computing resources could not brute force the secret.

Using a deterministic (yet known to the attacker) transform on the secret to get it to be 512 bits, then secret sharing that value is just fine for protecting the length of the secret.

Yes, you could encrypt the secret with a symmetric cipher like AES-256, though you still have to pad the secret to hide the length. Then you could share the encryption key with SSS. This does lose information theoretic security, however, as AES is not information theoretically secure. In practice I don't believe this to be a major issue, however.