I already did a lot of research and realized that clear information about "How to generate PDF/A-1a" or "...convert to PDF/A-1a" is really rare. I found some information to convert to PDF/A-1a via GhostScript, but I didn't make it to get it working. So, maybe there are some necessary conditions for the data missing in the first place. Conditions like propper metadata of the PDF, structured data for readability by a screen reader, alternative text for pictures, and a declaration of the given language of the text. I need a proper working GhostScript command with the corresponding gs version and the mandatory file conditions to generate or even convert to PDF/A-1a. PDF/A-1b means nothing to me because I'm already able to convert to that.
Thanks for any help.