how to change PDF default setting in Pages

Apple does not provide any export setting for PDF/A or PDF/X from any Pages version, or any Apple application's print or export panel. There is only an obscure PDF/A-2b from Apple's Preview application's File menu : Export… panel when you open an existing PDF in it. That is not a validated PDF/A-2b document.

If you already have Adobe Acrobat Pro (PDF Editor), you can open a reqular Pages PDF in it and then export as your choice of PDF/A or PDF/X.

You could also open any PDF in Affinity Designer 2 (free trial) and export it as PDF/A or PDF/X, again these are not validated PDF standard documents.

I would give more credence to using a current commercial PDF library in Affinity Designer 2 to export as PDF/X-3-2003 than the older Quartz filter PDF/X3-2002 (e.g. ColorSync) that one gets when using Preview's Export panel. Your choice. It still is an unverified PDF/X-3 document, and the printer may or may not reject it provided it passes their Adobe Acrobat Pro verification process.

The PDF/X3-2002 (PDF v1.3) and PDF/X3-2003 (PDF v1.4) are PDF standards specifications. See this article for what constitutes a PDF/X3 document. I don't see your point about "that's called a monopoly?"