Emailing out by group, category or individually

I am trying to develop a booking system where we use a pool of freelance interpreters externally. Within the board, I have added Emails & Activities add on. Unfortunately, you have to type or select each email address individually. This is to send an email out asking if they are available for a job at a certain time, date and location. Sometimes, we want to mass send to all interpreters, where data are kept in a different board. Sometimes we want to send to interpreters by specialism or by gender or by location and populate the “To” box with multiple email address when filtered like that. That is the flexibility we want to achieve when emailing out a booking request. Unfortunately, the Emails & Activities add on is rather basic. We don’t want to add them as guests because of GDPR - as they will see other bookings on the board. Does anyone have any suggestions? Thanks