You should be able to publish 8 values to ThingSpeak every 20 minutes (using snprintf and a webhook) and stay under 1 MB/ month without the need to Sleep the Electron. There are examples in this forum, but it appears you have a handle on combining the 7 samples into 1 publish. Reducing the number of decimal places will also help.
This thread may help, it’s where I learned.
https://community.particle.io/t/how-to-set-up-a-json-for-multiple-variables-in-a-webhook-integration/33172
I have several Electrons that publish 8 ThingSpeak fields at a minimum of 20 minutes (no sleeping), and any additional event triggered publishes. Those never use 2 MB/month.