I get what you’re trying to do - we ran into this issue as well with the Dawn theme. You can add alt text to the collection images in the Shopify admin, but that text does not show on the front end of the site, because the alt tags in the liquid templates are blank. No idea why this is, but if you run your Shopify site through an SEO tool, it will flag Collection images for not having alt text.
Here’s how we fixed it for the Dawn theme.
sections/main-collection-banner.liquid
This adds alt text to the collection image at the top of a Collection page.
Around line 37
Change
alt=""
to
alt="{{ collection.image.alt }}"
snippets/card-collection.liquid
This adds alt text to the Collection images wherever they appear in Collection lists.
Around line 53
Change
alt=""
to
alt="{{ card_collection.featured_image.alt }}"
View source after making these changes and you should be set.