EWCM is caused by a rise in oestrogen. Positive OPKs tell you you have a high level of LH. Your LH surge is triggered by the rise in oestrogen, but lags behind a few days, then ovulation is normally 12-24 hours after that surge.
Sperm can live several days, so the best odds of conception are actually to have sex BEFORE you ovulate. Generally it’s best to dtd when you have EWCM as sperm can live for longer when this mucus is present - as much as a week, but commonly 4 or 5 days. By the time you have an LH peak, you are often towards the end of your fertile window.
The only way to really know how your hormones work in terms of timing is to monitor EWCM, LH and temperature chart for when you actually ovulate. E.g. I usually have EWCM about 5 days pre-ovulation, and a positive OPK 1 or 2 days before. So my best chance for conception is actually between the two!