If you have studied the Bhagavad Gita, (as I have for more than 30 years) you will see that it is pure monotheism. God is described as the Supreme Person, and all living beings are described as His children.
The monothesm of the Gita is also clearly evident in the Puranas and Upanishads, specifically the Isha Upanishad, among others.
Nou doubt there are many "Hindus" (a culturally designative word invented by the Muslim invaders, describing those who lived on the other side of the Sind River) who haven't a clue about the deep truths of the Vedas, and think that ancestor worship and propiation of demigods is the entirety of the Vedic religion. This is unfortunate.