Unfortunately, neither SMS nor most messaging apps – including iMessage – allow you to send automated texts without your input to send each time.
There may be a workaround for what you are trying to accomplish and that is with the use of DO NOT DISTURB WHILE DRIVING MODE. Go to Settings-->Do Not Disturb and then scroll all the way down to the Do Not Disturb While Driving section. Set activate to Manually. Auto-Reply to All Contacts. And finally Auto-Reply where you can customize your auto response.
Now when you are in meetings then you can manually activate Do Not Disturb While Driving from the Control Center (Display control center by pulling down from the top right corner on your screen and tapping on the car icon). All incoming messages will then get your customized auto-response. Turn the mode OFF when you are done with your meeting(s).
Yes it is a workaround and you will need to remember to turn this mode ON and then OFF.
Furthermore, if you want to suggest this feature to Apple then please leave feedback here Product Feedback - Apple
Axel F.