How to convert (SQL) VARBINARY hashed password and salt columns to String for ASP.NET Core Web API

In my SQL Server legacy User table, I have the following columns (which I cannot change):

UserName: VARCHAR(50)
PasswodsHashed: VARBINARY
PasswordSalted: VARBIANRY

This table has thousands of records where the password was previously hashed using SHA512 via a stored procedure. I want to move the logic away from using a stored procedure into my ASP.NET Core 9 Web API.

Initially I am concentrating on the 'verify existing user/password' rather than the hash/create user.

I have the following models to handle the user details, login request & login response as follows:

public class User      // (SQL Server user table representation)
{   
     public string Username { get; set; } = null!;     
     public byte PasswordHashed { get; set; } // Should this be a BinaryData type?
     public byte SaltHashed { get; set; }
}

public class UserLoginRequest
{
     public string Username { get; set; } = null!;
     public string Password { get; set; } = null!;   
}

public class UserLoginResponse
{
     public User UserDetails { get; set; } = null!;
     public string Token { get; set; } = null!;
}

I am using the following SecretHasher (.NET 6+ from 2022) class which I recently found here

public static class SecretHasher
{
    private const int _saltSize = 16; // 128 bits
    private const int _keySize = 32; // 256 bits
    private const int _iterations = 50000;
    private static readonly HashAlgorithmName _algorithm = HashAlgorithmName.SHA256;

    private const char segmentDelimiter = ':';

    public static string Hash(string input)
    {
        byte[] salt = RandomNumberGenerator.GetBytes(_saltSize);
        byte[] hash = Rfc2898DeriveBytes.Pbkdf2(
            input,
            salt,
            _iterations,
            _algorithm,
            _keySize
        );
        return string.Join(
            segmentDelimiter,
            Convert.ToHexString(hash),
            Convert.ToHexString(salt),
            _iterations,
            _algorithm
        );
    }

    public static bool Verify(string input, string hashString)
    {
        string[] segments = hashString.Split(segmentDelimiter);
        byte[] hash = Convert.FromHexString(segments[0]);
        byte[] salt = Convert.FromHexString(segments[1]);
        int iterations = int.Parse(segments[2]);
        HashAlgorithmName algorithm = new HashAlgorithmName(segments[3]);
        byte[] inputHash = Rfc2898DeriveBytes.Pbkdf2(
            input,
            salt,
            iterations,
            algorithm,
            hash.Length
        );
        return CryptographicOperations.FixedTimeEquals(inputHash, hash);
    }
}

In my UserService, I have this issue in reading/converting the stored binary hashed pwd & salt columns, and matching them with the user's password input (string) to pass as a string to the Verify(...,..) method in the SecretHasher class shown above:

public async Task<UserLoginResponse> Login(UserLoginRequest logindetails)
{    
     // Fetch the stored  hash password and hash salt          
     var user = _context.TestUsers.FirstOrDefault(u => u.Username.ToLower() == logindetails.Username.ToLower());
    // I am having problems in the conversion on the next line from byte to string and I have tried several different ways using cast, ToString() etc etc..
    // I also need to concatenate PasswordHashed & SaltHashed to pass to the Verify method as a single input(?) ***
    var passwordHash = (byte)user.PasswordHashed; // +  byte (byte)user.SaltHashed; 

    string hashed = SecretHasher.Hash(passwordSalt.ToString());
    bool isPasswordCorrect = SecretHasher.Verify(logindetails.Password, passwordSalt.ToString());
    Console.WriteLine("isPasswordCorrect= " + isPasswordCorrect);
    // etc etc...
}

The usual error I get is

Cannot convert System.Byte to Byte (with the concatenation removed.

So the basic question is: how do I convert two binary columns into a one concatenated string?