Look at these that are available at Menards. Different CFM (cubic feet per minute of air movement.
These are made to be safe. I don't think your amount of dust is going to break these all that quickly.
I picture snipped the written features. States quiet operation.
Here is a link to read better if you need to.
The humidity is likely the same as outside. I’m in western Ohio and my humidity on this clear and sunny (but cold) day is 86%! So, you 70% may just be the general humidity there.
even so, enlarging the vents as described will work very well, and provide much benefit. Cover with HWC rather than chicken wire, as it is more durable.
I agree with closing the windows for the winter bc they are at roost level and the birds need well ventilated, but not drafty, and windows would provide a direct airflow in them with how they are situated. But, if you add more ventilation up high, the birds will not be in the draft.