You are not activating tagging in your example, so the alt key does nothing sensible; be aware that tagging is always for the whole document, so you can not tag only one part!
In your example you could tag your graphic like this (you can use tagpdf commands, the package is no longer only an experiment but the core of the tagging code of LaTeX, but you should know what you are doing ...):
\DocumentMetadata{pdfstandard=A-2b,lang=en,testphase=latest} % latest key is new and needs latex-dev
\documentclass{article}
\usepackage{graphicx, tikz}
\begin{document}
\includegraphics[alt={Example}]{example-image-a}
\bigskip
\tagstructbegin{tag=Figure,alt=a circle}\tagmcbegin{}%
\SuspendTagging{tikz}%
\begin{tikzpicture}
\draw (0,0) circle (1cm);
\end{tikzpicture}%
\par
\ResumeTagging{tikz}%
\tagmcend\tagstructend
\end{document}
It needs a bit fiddling to get the commands and their order right depending on if you are in a paragraph or not but generally it is not so hard. I do have a branch which automates that and which works reasonably well (and also adds a BBox which is missing here).
This is currently not in LaTeX oder LaTeX-dev as on one side I'm hoping for some better support on the tikz side (https://github.com/pgf-tikz/pgf/issues/1370) so that I can avoid too many patches.
The other problem is that I'm unsure how to handle examples like this:
Tomorrow is rubber ducky day
\begin{tikzpicture}
\node[draw=red]{Don't forget this, see \ref{sec:drawing}!}
\end{tikzpicture}
\todonotes{Don't forget this, see \ref{sec:drawing}!}
\begin{picture}(20,10) %
\put(0,0){Don't forget this, see \ref{sec:drawing}!}
\end{picture}
In such case the pictures are not illustrations and they should not be tagged as Figure. Writing the correct tagging code is not so difficult, but how should the interfaces for users and package writer like todonotes look like?
\begin{tikzpicture}[tag-nodes-as-text] %?
\begin{tikzpicture}[tagging-recipe=text] %?
And what should happen if one node in the picture is such a text but other nodes not?
