LO 6.3 provides PDF-export to PDF/A-2. That's great. Next step would be to implement PDF/A-3. I want to propose this.
Part 3 of the standard, published on October 15, 2012, differs from PDF/A-2 in only one regard: it allows embedding of arbitrary file formats (such as XML, CSV, CAD, word-processing documents, spreadsheet documents, and others) into PDF/A conforming documents.
Adding cc to Thorsten, he added support for PDF/A-2 export
Moving to NEW. I think it makes sense as an enhancement
This was merged in https://gerrit.libreoffice.org/c/core/+/84687. But that is just an additional entry to the combo box, as every PDF/A-2 is also a PDF/A-3
Now the question is, what LO should do additionally?
In theory LO's hybrid PDFs could be more PDF/A-3 compliant. Interestingly you can already select PDF/A + hybrid PDF, but that is invisible in a PDF viewer.
I can imagine embedding additional data via UNO would make some sense, not sure about GUI stuff.