66% is fine for a grow in my opinion, i use humidity domes until they sprout and after that my average rh is 65-70 until I need it drier. I also will run my humidifier right next to the plants with no circulating air for the first week. I’ll set the tent rh real high like 80% and the fan set to just 1 on power speed. This will help me stay in the 70s til not needed.
But I think you should run whatever lighting you want, keep the humidifier on 24/7, and don’t worry about what the probe says. When your pot filled with wet substrate hits the scene it’ll provide what you need. This might be disputed but I’m only speaking from experience, I can’t give you science.