Thanks for all feedback. Bottom line: to use Affinity tools I need to:
a) have to spend some extra bucks on a third-party software ONLY to convert to PDF/X1a-2001; or
b) change my print shop to one that's use a more modern profile; or
c) quit using Affinity tools at all, spend money with a designer to make my jobs (and have a tool capable of export to PDF/X1a-2001 like Adobe or Corel ones) because I will not pay for a signature to use the tools once or twice a month or less than this.
Either alternatives are not good to me but meanwhile Seriff looks do not care to this I have no choices. At least they can put some kind of blog post about this explaining if is hard to make this, it is expensive, etc. I am not the first one to ask about this conversion.
Thanks again for all feedback. Good day everyone and stay in peace.