You are correct. Unfortunately, automations can not trigger any apps/blocks. It’s one of the many disconnects in the Airtable platform.
However, you can manually click on a button field to open the SendGrid app for you, and here’s the cool thing: It will automatically populate the SendGrid app with only the record you just clicked on. Then you can manually click on the “Send” button. So this is a manual process, but it still works “relatively well”.
The only problem here is that it might require up to 7 clicks to accomplish this (depending on how wide your apps panel is, and whether it’s showing or not): You have to click the button to open the SendGrid app, then you might have to click on a button to make the SendGrid app go full screen (or widen the apps panel), then you have to click on a button to “preview” the email, then you have to click on a button to “Send” the email, then you have to click on the “Done” button after it has finished sending, then you have to click on a button to make the SendGrid app exit full screen mode, then you have to click on a button to hide the apps sidebar.