Using a 100% duty cycle is a bad idea as you’ll be blocking the signal for any other device using the same frequencies.
The spreading factor/datarate is a trade off:
Low SF will allow larger payloads to be transmitted in a shorter time but over a much reduced distance.
High SF will require smaller payloads to be transmitted over a longer time but the range will be much greater.
You pick whats best for your application and use-case.
Andrew