I found on here the suggestion of Export As Images » Extensions
But that only exports each slide to an image - it doesn’t give the option (that I am aware of) to generate an image for each transition stage.
So if a slide has 3 bullet points I would want the following: an image with no bullet points, an image with the first bullet point, an image with the first and second bullet point, and an image with all three bullet points.
It is possible to create such a PDF using this extension: GitHub - monperrus/ExpandAnimations: LibreOffice/OpenOffice.org extension to expand animations before exporting to PDF. Looking for maintainers. · GitHub
Once the PDF is created, you can use a command-line tool like pdftoppm to quickly extract the images.
However, I have found the extension to be unreliable (e.g., it can make mistakes with text box size). It also gets page numbers wrong.
The solution that ended up working for me was to go through the slides and manually screenshot each transition using the printscreen button on my keyboard. A manual solution but still relatively fast.