https://tex.stackexchange.com/q/612764/35864

seemstowork.tex

\documentclass[british]{article}
\usepackage[T1]{fontenc}
\usepackage{babel}
\usepackage{csquotes}

\usepackage[backend=biber, style=authoryear]{biblatex}

\newrobustcmd*{\nextcitefullname}{%
  \AtNextCite{\DeclareNameAlias{labelname}{given-family}}}

% Not sure if I should be using \DeclareCiteCommand here instead.
\NewDocumentCommand \citeauthorfullname { s } {
    \nextcitefullname
    \IfBooleanTF #1 { \citeauthor* } { \citeauthor }
}
\NewDocumentCommand \Citeauthorfullname { s } {
    \nextcitefullname
    \IfBooleanTF #1 { \Citeauthor* } { \Citeauthor }
}
\NewDocumentCommand \textcitefullname { } {
    \nextcitefullname
    \textcite
}
\NewDocumentCommand \Textcitefullname { } {
    \nextcitefullname
    \textcite
}

\addbibresource{biblatex-examples.bib}

\begin{document}
Lorem \autocite{sigfridsson}

\citeauthor{sigfridsson}

\citeauthorfullname{sigfridsson}

\printbibliography
\end{document}

slightlysimplified.tex

\documentclass[british]{article}
\usepackage[T1]{fontenc}
\usepackage{babel}
\usepackage{csquotes}

\usepackage[backend=biber, style=authoryear, natbib]{biblatex}

\newrobustcmd*{\nextcitefullname}{%
  \AtNextCite{\DeclareNameAlias{labelname}{given-family}}}

\newcommand\citeauthorfullname{%
  \nextcitefullname\citeauthor}
  
\newcommand\Citeauthorfullname{%
  \nextcitefullname
  \Citeauthor}
  
\newcommand\textcitefullname{%
  \nextcitefullname
  \textcite}
  
\newcommand\Textcitefullname{%
  \nextcitefullname
  \textcite}

\addbibresource{biblatex-examples.bib}

\begin{document}
Lorem \autocite{sigfridsson}

\citeauthor{sigfridsson}

\citeauthorfullname{sigfridsson}

\citeauthor{aksin}

\citeauthor*{aksin}

\citeauthorfullname{aksin}

\citeauthorfullname*{aksin}

\printbibliography
\end{document}