Hi Marie,
One thing you can do to improve this, is to add the buttons to a button set. This ensures that only one button is selected at a time.
To add the buttons to a set, select all the buttons > right click > Button Set > Button Set 1.
I would also recommend adding a comma before the button state. This helps users differentiate between the button labels and it's state, for example "1) Legislation, expanded". The comma adds a short pause to the screen reader.
The other issue is that when you close any of the layers using the (x) close button, the button you have just selected remains selected. This would need to be reset. I would actually consider removing the close layer button as it seems to be sending the reading focus back to the top of the slide. It will be a better experience for screen reader users without the close button.
In the example I have provided, I've also hidden the "selected" button from the base and added a duplicate of the button to the open layer with the "selected" state showing. This ensures that when the layer is opened, it will read the "expanded" state of the button.
Hope this helped you out. It can be difficult figuring out the best way to structure content for accessibility in SL.
Cheers,
Sam