有几种可能性来解决你的问题。
可以像这样更改Home表单构造函数

public Home(string portName, int baudRate)

从第二种形式调用它

Home home = new Home(
    comPort_comboBox.Text,
    Convert.ToInt32(baud_rate_combobox.Text));

有了这个，你可以直接在第三种形式内管理串行端口（因此打开它内部的端口并设置事件以接收其中的数据），而无需共享其示例（可能导致其他问题恕我直言）。
如果您需要以某种方式管理串行端口，即使是在第二种形式中，那么您可以更改第三种形式的构造函数来传递它：

public Home(SerialPort serialPort)

并将其命名为Home home = new Home(serialPort1);

1.您可以在SerialPortConfig.cs中使用save the serial port config，

1. Read the serial port config从SerialPortConfig.cs，而您在MainForm.cs中
   SerialPortConfig.cs

using System.IO.Ports;

namespace SerialPortNs
{
    public class SerialPortConfig
    {
        private static readonly SerialPortConfig _instance = new SerialPortConfig();

        private SerialPortConfig()
        {
            //
        }

        public static SerialPortConfig Instance()
        {
            return _instance;
        }

        public string? PortName { get; set; }
        public int BaudRate { get; set; }
        public Parity Parity { get; set; }
        public int DataBits { get; set; }
        public StopBits StopBits { get; set; }
        public Handshake Handshake { get; set; }
    }
}

SettingsForm.cs

using System.IO.Ports;

namespace SerialPortNs
{
    public partial class SettingsForm : Form
    {
        public SettingsForm()
        {
            InitializeComponent();

            // save serial port config
            SerialPortConfig config = SerialPortConfig.Instance();
            config.PortName = "COM1";
            config.BaudRate = 9600;
            config.Parity = Parity.None;
            config.DataBits = 8;
            config.StopBits = StopBits.One;
            config.Handshake = Handshake.None;

            // open MainForm
            Application.Run(new MainForm());
            
            // hide SettingsForm
            this.Hide();
        }
    }
}

MainForm.cs

using System.IO.Ports;

namespace SerialPortNs
{
    public partial class MainForm : Form
    {
        private SerialPort _serialPort = new SerialPort();

        public MainForm()
        {
            InitializeComponent();

            // read serial port config
            SerialPortConfig config = SerialPortConfig.Instance();
            _serialPort.PortName = config.PortName;
            _serialPort.BaudRate = config.BaudRate;
            _serialPort.Parity = config.Parity;
            _serialPort.DataBits = config.DataBits;
            _serialPort.StopBits = config.StopBits;
            _serialPort.Handshake = config.Handshake;

            if (_serialPort.IsOpen)
            {
                _serialPort.Close();
            }
            _serialPort.Open();
        }
    }
}

Program.cs

namespace SerialPortNs
{
    internal static class Program
    {
        /// <summary>
        ///  The main entry point for the application.
        /// </summary>

        [STAThread]
        static void Main()
        {
            ApplicationConfiguration.Initialize();

            // Open SettingsForm on startup
            Application.Run(new SettingsForm());
        }
    }
}