Hi @emilyruby and welcome to the forum!
I know the feeling as I've worked with a client that has multiple apps connecting to shopify via Graphql and we need to add transformers to each query.
Something that comes to mind is that you could set up a global function and then in your transformer simply pass data as a parameter. You would still have to call the function in each query, but i guess it is better than copying the transformer every time. And if you have a need to update the transformer, you can do it only once in the function.
Update: The closed beta for resource functions and sync functions has concluded, and the feature is not being enabled for additional organizations. No further improvements or bug fixes will be made. See the Legacy: Resource and sync functions docs for details.