I just acquired a second fan, spliced it right into the first fan's plug, and zip-tied it to the radiator. Been running it for 1000km now, 50% of the off-road. All in about 30 deg C heat. I actually found I was getting to 5 bars earlier
than before, and staying there, even once I was up and running at a good 60 kph
Fans were coming off and on quite frequently.
I figured that when the second fan was off, it was actually impeding the airflow where it used to just go straight through once you go moving. When it heated enough, the fans kicked it, cooled it just below the threshold (which is still in 5 bar territory), then turned off again, blocking the airflow. So it just kept bouncing around the 'low' 5 bar mark.
Of course, I just noticed a fair part of my grill was packed with mud....that probably didn't help.