I'm Ava, I help moderate the Community Forums here at Shopify. I'm sure I can help here!
We have a great guide here in our help docs on how to add alt-text to your product images, and how you can add alt-text to your theme images directly in your Theme Editor. These images include ones you have added to a slideshow, image gallery, or even to certain pages.
Updating the alt-text for each individual product can take a lot of time to do manually, so I'd recommend speeding up the process by using an app. You mentioned this is something you would like to do in bulk without using one Graphql query, so these are some apps that are highly rated by other Shopify merchants:
With these apps, you can update the alt-text for all of your online store's images in bulk and even receive recommendations on how to make your alt-text more effective for SEO. They also have the ability to automatically apply alt-text based on the product title and description. Feel free to check them out and see which one suits your needs the best. To get you started, I'd recommend checking out this post in the Shopify UX blog, to learn more about what alt-text is, why it's important, and how you can write effective content for your images.
Working on your images alt text is a great boost to your SEO, and it also makes your website more accessible for customers who are visually impaired and use assisted technology to have content read aloud to them. Can I ask if you're taking any other steps to increase your site's SEO?
Happy to chat further!
Social Care | help.shopify.com