If you have the resources it's worth fixing the motorized vents. Combined with a thermostat they will give you the unattended cooling that you are asking for.
I agree with
@subarctic that you need to refurbish the vent and motor system. From 1,500 miles away I'm guessing the motors keep failing because they are burning out trying to push open the old, likely unlubricated and maybe rusted, hinges and bearings. You might also look to see if any of the gears have been stripped or if any of the parts are bent or damaged.
In addition to fixing what you have you can also throw some shade cloth over the top of the greenhouse. It looks like there might be some on there already but if your greenhouse is too hot you might want to use a more opaque shade cloth than you have currently.
