Oh man, I can go into a bit about the humidity control but without really playing with the setup you'll have to experiment a bit for yourself.
But yes humidity would fluctuate if exhaust were an on/off thing, thats not necessarily bad if its just cycling pretty often all times. But like leaving it go up all night, then exhausting it back to dry can be an issue. but if you're exhausting it all to like 10% all the time its not really helpful at either. So at this point with small plants in there you might try just not exhausting and opening the tent once in a while, does it really get that high? I don't see those babies making it jump past 60%s very quick. And they really don't need that much air exchange, for like what 50 leaves total.
When they start getting more leaves to transpire you'll notice the humidity levels go up much more, and the low exhaust setting might just get it closer to your goal without overdrying. Then they'll get into flower and you'll want to bring that down more and you can turn er to high, and with all the plants in there it'll still stay up a little bit.(won't get bone dry so fast) Does that make sense?
I think my biggest hangup on some growspaces is actually all that excess power. Running dehumidifiers, humidifiers, ac and heat in the same space so they're always cycling back and forth with each other and all that conditioning is just getting blown out the window anyway. Lungrooms...