In TableLayoutPanel if Dock = Fill and AutoSize = true then last column/row occupy all remaining space.

foreach (CustomButton b in buttons)
    {
        b.Dock = DockStyle.Top;
        AddRow(b);
    }
}

private void AddRow(CustomButton b)
{
    int rowIndex = AddTableRow();
   tableLayoutPanel.Controls.Add(b, 0, rowIndex);
}

private int AddTableRow()
{
    int index = tableLayoutPanel.RowCount++;
    RowStyle style = new RowStyle(SizeType.AutoSize);
    tableLayoutPanel.RowStyles.Add(style);
    return index;
}

tb1.RowCount = tb1.RowCount - 1
tb1.RowStyles.RemoveAt(tb1.RowStyles.Count - 1)
tb1.AutoScroll = False
tb1.AutoScroll = True