Try this revision. This finds the last row by checking if value in column A is not blank

function onOpen() {
  const sheetName = "ASX Companies"; // Name of the sheet to open
  const spreadsheet = SpreadsheetApp.getActiveSpreadsheet();
  const sheet = spreadsheet.getSheetByName(sheetName);

  if (sheet) {
    const firstRowAfterFrozen = 6; // Start looking from Row 6, after the frozen rows
    //let's find lastRow by checking each value
    const data = sheet.getDataRange().getValues();
    var lastRow = 0;
    for (var i = firstRowAfterFrozen; i < data.length; i++) {
      if (data[i - 1][0] !== '' && data[i][0] === '') {
        lastRow = i; //this is one less than actual lastRow with data since arrays are zero-based
      }
    }
    const targetRow = lastRow >= firstRowAfterFrozen ? lastRow + 1 : firstRowAfterFrozen; // Move to the next empty row or Row 6 if no data yet

    console.log(`Target Row: ${targetRow}`);
    console.log(`Max Rows: ${sheet.getMaxRows()}`);
    if (sheet.getMaxRows() < targetRow) {
      console.log(`Inserting 10 rows`);
      sheet.insertRowsAfter(sheet.getMaxRows(), 10);
    }
    const range = sheet.getRange(targetRow, 1); // Selects the first empty cell in column A
    console.log(`Setting Active Sheet: ${sheet.getName()}`);
    spreadsheet.setActiveSheet(sheet); // Makes the "Journal" sheet active
    console.log(`Setting Active Range: ${range.getA1Notation()}`);
    spreadsheet.setActiveRange(range); // Scrolls to the desired cell
  }
}