I don't know anything about hyperxmp, but for sure pdfx package loads xmpincl to handle the expected XMP metadata.
Your problem could arise from a slight syntax confusion.
1. Your approach
\hypersetup{
pdftitle={\@title},
pdfauthor={\@author},
pdfcreator=pdfLaTeX,
pdfproducer={\@author},
}
should work, as long as :
- you have loaded
hyperref
- you don't have loaded
pdfx (see pdfx manual page 17)
- you did'nt use
\maketitle which by default empties both \@title and \@author
A MWE :
\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage{blindtext} % for example text only
\title{A simple title}
\author{A cute author}
\usepackage{hyperref}
\makeatletter
\hypersetup{
pdftitle={\@title},
pdfauthor={\@author},
pdfcreator={pdfLaTeX with hyperref},
pdfproducer={\@author},
pdfsubject={Some answer about metadata},
pdfkeywords={LateX hyperref},
}
\makeatother
\begin{document}
\blindtext
\end{document}
and the meta data shown in PDF-Xchange and Adobe Reader:

.
2. Another option, without hyperref , using pdfLaTeX's command \pdfinfo, used as:
\pdfinfo {
/Title (\@title)
/Author (\@author)
/Subject (\@suject)
/Keywords (\@keywords)
}
The MWE :
\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage{blindtext}
\title{A simple title}
\author{A cute author}
%\usepackage{hyperref}
\makeatletter
\pdfinfo {
/Title (\@title)
/Author (\@author)
/Subject (Some answer about metadata)
/Keywords (LateX hyperref)
}
\makeatother
\begin{document}
\blindtext
\end{document}
and the result:

3. if you load pdfx, the metadata set relies on the \jobname.xmpdata file
which could look as (see pdfx manual page 6):
\Title{Baking through the ages}
\Author{A. Baker\sep C. Kneader}
\Language{en-GB}
\Keywords{cookies\sep muffins\sep cakes}
\Publisher{Baking International}
\CoverDisplayDate{1er\ avril\ 1999}
\CoverDate{1999-04-01}
and as said in 1., \hypersetup method is disabled.
A third MWE :
\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage{blindtext}
\usepackage[a-3b,pdf17]{pdfx}
\title{A simple title}
\author{A cute author}
%\usepackage{hyperref}
\begin{filecontents*}{\jobname .xmpdata}
\Title{Another simple title}
\Author{A cute author}
\Subject{Some answer about metadata}
\Keywords{LateX \sep hyperref}
\end{filecontents*}
\begin{document}
\blindtext
\end{document}
and its result:

Please notice that:
- I used
pdfa-3b as pdfa-3u is not suitable according to @UlrikeFischer answer
- In the 3rd scheme (pdfx) the title ,is the one written in the
.xmpdata, independently of \title in the main document
- The keywords creator, producer and others of the same kind are usually automatically set by
pdflatex and pdfx, and could require much more effort if you really want to customize them.
pdfaconformance? – tecosaur Sep 30 '19 at 16:53