This suggests that you’re using Gmail and not gsuite, which is against the rules. Lots of people, topically people who don’t know what system administration is, try to use Gmail and it is a violation of the terms of service. Telling people that is a good idea, and not mean or rude.
But you’re using gsuite, which is totally fine. That was impossible to tell from your original post. So that’s why it seems like you’re being treated so unfairly.
Except Gsuite doesn’t actually work on a forum with much traffic, as you describe. You are asking for a new feature so that Discourse will somehow work with a mail service that limits you to 2500 messages per day.
It’s possible, but won’t be very easy, to create a plugin to do that. I suspect that it would be several days work for someone familiar with discourse development.
The solution is to use a mail service that will deliver the amount of mail that you need to send.