When and how to start lowering HIGH humidity levels for seedlings?

0.04% of air is c02 , the shit the leaves need. More water = less everything else

High humidity is only needed to break the shell as soon as the cotyledons get things down to a reasonable level, leaves have tiny lungs on the back of leaves called stomata. high RH above 70-75% will force the plant to close its lungs to prevent "choking" on high water levels. air is for breathing, moisture above 77 indoors is suicide for amatuers, what your tiny teeny plant needs most is constantly replenished airflow, whilst keeping moisture in the roots where its needed, get an inorganic mulch, rocks pebbles or somewhat to keep the top layer moisture locked in a little longer. At night the temps cool, and air holds a lot less capacity of moisture when it's cold. your 70% daytime can easily jump into 80-90 at night which is suicidal for any indoor grow.

You should give them more light, because they look weak. Dont worry too much.

You can lower it down to 80% then 75% till your hit 65%, low n slow should be fine. Good Luck!

60-70% is ideal. Honestly anything above 50% will work.