I send 88 emails per month, and I find that I can get 29-31 emails out in that 6 min window. With only an A/B/C list, I can only add 2 more players before I have to start sending 1-off emailsOne way around it is to use the Siri Shortcut on an iPhone because that takes the processing of the messages out of the Google Script and on to the iPhone.
The other way is to try and optimize the code t be more efficient about how it runs so t that more emails can be sent within the 6 minute window. I've already done some work in that area in v4.0 but I don't know what that will translate to in terms of how many emails you can push through in the 6 minute window. Google Script is just very slow anytime you ask it to access a range in a sheet. I've already tried to consolidate those lookups to as few as possible.
Out of curiosity, how many emails are you looking to send?