So I could succesfully implement a different method from this guy: https://answers.unity.com/questions/1378780/save-timer-highscore-1.html Thanks everyone for your patience and help!

So I could succesfully implement a different method from this guy: https://answers.unity.com/questions/1378780/save-timer-highscore-1.html Thanks everyone for your patience and help! Currently this is able to save my highscore if I hit the restart button to reload my scene. If you stop playing and hit the play again however it resets to zero, so there's a chance this isn't what others are looking for.

Modified code:

public TextMeshProUGUI scoreText;
public TextMeshProUGUI highScore;
float score;



// Start is called before the first frame update
    void Start()
    {
        StartGame();
        highScore.text = PlayerPrefs.GetFloat("HighScore", 0).ToString();
    }
    // Update is called once per frame
    void Update()
    {
        if (isPlayerAlive == true)
        {
            score += Time.deltaTime * scoreMulti;
            scoreText.text = score.ToString("Score: 0");

            if (score > PlayerPrefs.GetFloat("HighScore", 0))
            {
                PlayerPrefs.SetFloat("HighScore", score);
                highScore.text = score.ToString();
            }
        }
    }