Given that the observed conversion rates are within an acceptable rate, consider increasing your CPI once each creative pack has reached at least 5000 impressions, then evaluate for 12 -14 hours. This is when our delivery system takes these impressions into account (when we need to retrain our models to ensure that they are actually in production).
Avoid taking the bid down too much from this optimized level, but take it down to a CPI range acceptable to you.