Provide an "aria-label"to "carousel-inner", explaining you are interacting with a slider, a set of pictures. As with the Carousel Header, the Carousel is annouced as a "list box" by VoiceOver because of the "role=listbox" attribute in the markup, you can press VO+Shift+Down Arrow to start interacting with it.
Add an "aria-live" attribute to "carousel-inner", so when you press one of the buttons, the next image is read aloud automatically.
Remember image captions for SRs should start with something like "Image of a...", maybe we can add something preceding the caption. Maybe just "Picture:".
A play/pause button. As this is a Bootstrap Carousel, investigate on how to build that feature, maybe there's something in the Bootstrap docs. Toggling the play/pause button should change the aria-live attribute from "off" to "polite" respectively. "Off" when playing, "polite" when paused.
The text for the buttons "Previous" and "Next" should be more descriptive, e.g.: "Previous slide".
Make sure the Gallery block sounds good and makes sense in a Screen Reader, on its three different styles.
Carousel Gallery
Three-columns Gallery
Grid Gallery
Potentially affected repositories: planet4-plugin-gutenberg-blocks
Testing
Add the Gallery block to a page and test it with a screen reader. You can check the docs for more details on how the block works.