All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
A woman knows the purity (Taharah) after menses by one of two signs: Either complete dryness, and it is inspected by inserting cotton into the place, and which comes out clean without any trace of blood, yellowish or brownish.
The second sign is white Al-Qassah, which is a discharge that tends to be whitish in color, and you mentioned in the question that it usually comes to you after your menstrual blood has ended.
Based on that, if you see dryness, then you have become pure (from menses), and you do not have to delay the Ghusl until you see the white discharge.
Allah knows best.