Would all customer records be updated with the same tag, would a subsection of customers be updated with the same tag, would customer records be updated with different tags, etc.? It's doable in either REST or GraphQL API's. Going through the REST API it would likely involve a two-step process: 1) pulling the customer id's you need updated, 2) iterating through each customer and pushing the updated tag for them.
A subsection of customers based on email domain would be updated with the same tag, but they might already have different tags assigned to them and I don't want to remove them just add the new one. Yes I know how to update them through the REST api one by one but the request limit is way too small for that.