I had forgotten about the LTE transport. I was thinking that your sensor boards would be talking WiFi to something on your local net and then that data could be pushed to the external web site. MQTT would work well there.
But if you're sending data direct to the website via SMS, I don't see the benefit of adding a local broker. You could get it to work - for example you could have a master board that all the SMS goes to and then have it push sensor data into MQTT, but it's just extra complexity for not much I think.
Unless you have something local that's going to subscribe to the broker on the Pi, I'm not sure it'll help much apart from some learning.