You could extend ActorsExecutingService and override GetFormSubmissionActors.

In the override, you could call:

base.GetFormSubmissionActors(submission, formContainer, formIden, request, response, isFormFinalizedSubmission).ToList();

Then check if the returned list contains a SendEmailAfterSubmissionActor. If it does, check the submitted data, and update the EmailTemplateActorModel's property ToEmails, and return the list.

Finally tell Episerver to use your implementation instead of ActorsExecutingService. Can be done using StructureMap.