Marketing automation does not have a way to send bulk email campaigns via their action. So you need to send them 1x1. That is possible, but you’ll need to use an action to “Get” the customers and then a “For each” action to loop over the customers (caveat - this action has a limit of 100 at a time).
The “Get customer data” action does not yet support segments, because the API doesn’t yet. But you can query by metafield values, if those metafields are indexed. This shows the syntax for the query https://shopify.dev/docs/apps/build/custom-data/metafields/query-by-metafield-value#querying-products-by-field-value
Something like:
metafields.custom.material:"cotton"