since you plan for two non consecutive days at uo, thursday and saturday, i assume you have motherland plans for that friday.
if that is so, a two night stay at the darkside wouldn't work with your plans.
when the prices are $31.99 that is an indicator of a lower crowd day.
at the end of august, the hours are changing for park closing and there are less crowds.
saturdays are always busy no matter what park you go to.
that's a given.
never buy your epp in advance, especially in an off time period. typically low attendance period)
go to the parks.
if the lines are over 20 minutes and you have limited time, buy the epp then.
you can only use them one time only per ride/attraction.
do as much as you can with the shorter lines when you arrive.
then buy the epp if lines are too long for you.
repeat the rides you did before w/ express..
if it rains, the park thins out.
if you have done about 3/4 of the park without many waits, epp is a waste.
the epp is date and park related.
there are no refunds, none..
it would be a waste to buy them in advance when you find out later you didn't need them.
only exception is the price of the epp can change.
they are cheap right now for your dates.
if universal changes the price up wards on the epp, it means they feel the crowds will be greater than previously expected.
epp's do not run out in your time period.
then can be bought in the park at the major merch store or at guest services anytime.
if you go thursday and decide to repeat the parks on saturday, that would be the day to 'consider' the purchase of epp.
but, again, wait until you are in the park and can judge the crowds and lines.
as others have posted, staying onsite is the best way to handle the parks, crowds and your time.