This method will not preserve the document structure and will result in a PDF document with no tag structure. Starting with an accessible slide deck, a goal when exporting to PDF is to do so in a way that preserves the structure of the PowerPoint, including slide titles, reading order, alternate text for images, and markup that explicitly identifies lists, tables, document language, and other content that is important for accessibility.

Make sure to follow the core steps for accessibility as outlined on the PowerPoint presentations page. Use InDesign for ideas, but take advantage of the tools the other app offers, because the two softwares were planned for very different tasks.The first step in creating an accessible PDF from PowerPoint is to ensure that the original slide deck is accessible. You will never be able to get the client to use things exactly as you explained, but if you have a template they will have less chances to mess up. In the end it will be about using PP to get all those assets together, to create a template that you have designed previously. The most work you can produce outside of it, IMO, the better. Powerpoint is quite powerful, but it's also a bit heavy. Create and import videos files to replace complex animations.

Create and atomise elements you want to re-use (bullet list styles, text styles, anything really).Prepare graphs and graphic assets as transparent PNGs.Adjust image formats and sizes so they doesn't have to scale weirdly (export them in the correct quality and size from the start).I personally spend almost as much time preparing the assets for the presentation as I do preparing the presentation file itself. A template contains everything from different covers, layout and styling options, to (if you want) guidelines in regards to animations (this would have to go separate).

Since re-usability seems to be an important point here, I would consider preparing a Powerpoint template for your client. As you mentioned, your client might want to change things, so the format should be a popular and easy to edit one. If the presentation needs to be done in Powerpoint, I wouldn't try to use a different platform.

Indesign export powerpoint