Hello soulcalm17

[Paleophyte]

In Greece and around the Aegean it seems pretty clear that there were lots of gods, and that gradually Zeus/Jupiter came to be thought of as dominant -- almost monotheistic in a Christian way. Of course they were comfortable with symbolic or allegorical representations, so there were always variations, but the later Roman religion is pretty monotheistic. 

Very much no. Zeus was first amongst equals at best and had little to no power in his brothers' domains. The Greek religions, which the Romans co-opted, were never even henotheistic or monolatric, much less monotheistic. We have that from both Greek, Roman, and early Christian sources.

In Hindu scripture, actually God is depicting as uniquely one and unseen entity. Not many people know about it I guess. But it was mentioned in Upanishad. Hinduism start from about 3000 BCE (some says 8000 BCE and even timeless according to Hindus people) and as time continue, it goes to polytheistic style and had millions of God.

You seem to have that front to back. The early Vedic religions (~2nd millennium BCE) were polytheistic, animistic, and shamanistic in many respects. These eventually transition into Brahmanism around the beginning of the first millennium CE, which is what you find in the Upanishad. That underwent further syncretic fusion with a whole host of local religions to produce modern Hinduism, which is incredibly heterogenous. Importantly, Brahman isn't a deity in the sense that any Abrahamic religion would employ the term but rather a metaphysical concept. It's the "ultimate reality" from which the universe originated and into which it will one day return, similar to the "raw firmament" of western religions but without the requirement of any deity to shape it. At no point to you get anything even vaguely like monotheism.