먼저 접근한 방식은, 원하는 해상도를 일일히 하나하나 버튼에 넣는것이었다.
하지만 인터넷 서핑도중 더 좋은 방법을 찾아내어 사용함.

드롭다운 및 토글!

// .....

[SerializeField] GameObject changeScreenImg;
[SerializeField] Toggle fullScreenToggle;
[SerializeField] TMP_Dropdown changeScreenDropdown; //tmp드롭다운 사용
FullScreenMode fullScreenMode;
private List<Resolution> resolutions = new List<Resolution>();
private int resolutionNum;

// ....

#region ChangeScreenOption

private void InitScreenResolution() // make Screen Resolutions and add it into Dropdown
{
    resolutions.AddRange(Screen.resolutions);
    changeScreenDropdown.options.Clear();

    int screenOptionNum = 0;
    foreach (Resolution item in resolutions)
    {
        TMP_Dropdown.OptionData option = new TMP_Dropdown.OptionData();

        option.text = item.width + " x " + item.height + " " + item.refreshRateRatio + "hz";
        changeScreenDropdown.options.Add(option);

        if (item.width == Screen.width && item.height == Screen.height)
        {
            changeScreenDropdown.value = screenOptionNum;
        }
        screenOptionNum++;
    }
    changeScreenDropdown.RefreshShownValue();

    fullScreenToggle.isOn = Screen.fullScreenMode.Equals(FullScreenMode.FullScreenWindow) ? true : false;
}

public void OnClickChangeScreenDropdown(int x) //get num of choosen Screen Resolution from Dropdown
{
    resolutionNum = x;
}

public void OnClickFullScreenToggle(bool isFull) // check Screen is already fulled or not
{
    fullScreenMode = isFull ? FullScreenMode.FullScreenWindow : FullScreenMode.Windowed;
}

public void OnClickAdmitScreenChangeBtn() // Click Admit Btn, set up Screen Resolution and Fulled Window
{
    Screen.SetResolution(resolutions[resolutionNum].width, resolutions[resolutionNum].height, fullScreenMode);
    changeScreenImg.SetActive(false);
    menuCloseBtn.SetActive(true);
}
#endregion

뭔뜻인지 이해는 안가지만, 복사 붙혀넣기 하면 이해가 될것이다..
다만
드롭다운이나 토글에서 해당 함수를 OnClick()메서드에 넣어줄때,
Dynamic에서 넣어줘야한다!