Hi,
To change the controls background color to transparent you need not set
ControlStyle SupportsTransparentBackColor to true . Moreover,
inside the OnPaint method,  e.Graphics.Clear(Color.BackColor);  is also not
required
you can instead write

base.OnPaint(pevent);    in your OnPaint method and it will work fine.

I have tried the following code and it works fine for me

public class rb : RadioButton    
        {
           
            protected override void OnPaint(PaintEventArgs pevent)
            {
                base.OnPaint(pevent);

            }
        }
       

        private void Form1_Load(object sender, EventArgs e)
        {
            rb r1 = new rb();
            r1.BackColor = Color.Transparent;
                       this.Controls.Add(r1);
                       
        }