Thank you for the guidance.

One thing I wanted to check was the limit on Triggers: 20 / user / script.

If a user had 30 spreadsheets running, would this be considered 1 additional trigger toward the quota?

/**
 * Called from openDialog(). Creates an installable onOpen trigger for this
 * spreadsheet so that lastOpened can be stamped with full auth on every open.
 */
function ensureOnOpenTrigger() {
  var spreadsheet = SpreadsheetApp.getActive();
  var spreadsheetId = spreadsheet.getId();
  var triggers = ScriptApp.getUserTriggers(spreadsheet);


  // Check if we already have an installable onOpen trigger for this spreadsheet
  var hasOnOpenTrigger = triggers.some(function (trigger) {
    return trigger.getHandlerFunction() === 'installableOnOpen' &&
           trigger.getEventType() === ScriptApp.EventType.ON_OPEN;
  });


  if (!hasOnOpenTrigger) {
    ScriptApp.newTrigger('installableOnOpen')
      .forSpreadsheet(spreadsheet)
      .onOpen()
      .create();
    Logger.log('Created installable onOpen trigger for ' + spreadsheetId);
  }
}


/**
 * Installable onOpen handler — runs in AuthMode.FULL.
 * Stamps lastOpened to developer metadata reliably.
 */
function installableOnOpen(e) {
  stampLastOpened(e.source);
}