We are trying to pick a maximum motor speed and could use some guidance.
Our application will have a 30% duty cycle and run 24/7 for several years. It is remote and solar powered, so power efficiency is important. If our typical motor speed is X RPM, how high should we go on our maximum speed? 2X, 4X, 8X? Is running a brushed motor at much less than full speed less efficient?

Applying a lower PWM to a motor is equivalent to applying a lower DC voltage. E.g. 24V battery and 50% PWM is equivalent to having a 12V battery directly connected to the motor.

Higher or lower voltage do not make much difference in itself. The effect of higher voltage is higher current and higher speed and these are what matters more.

HIgher current causes heat. Heat causes increased winding resistivity, wich in turns causes inefficiency and more heat.

Higher speed causes higher friction at the bearings and the brushes.

The motor maker may be able to give you guidance. Otherwise, you may need to perform your own tests.