What contradictions? I have never felt any disharmony between the two to be honest.

Animal Sacrifice and so on? Mostly referring to a lot of ethical contradictions more than anything. As well, the Buddhism framework of how the universe is structured differs significantly from the Vedic view.

It's important to understand though that there is no single Vedic religion, and "Hinduism" isn't a single religion. Hinduism refers to a number of different religions that all pull from different parts of the Vedas in different ways. In this way, there is no cohesive religious belief throughout India, as there are many different Vedic traditions, many of which differ greatly from each other.

So yes it's likely there's a few which parallel Buddhist ideas, but not all of them.

The Upanishads are cool but it's important to realize they're also not a cohesive text. "Upanishads" translates to "Secret Teachings", or more accurately, "Sitting at the Feet of an Illuminated Teacher". In this way the Upanishads are a collection of insights from numerous different advanced sages from history, some which were likely more knowledgeable than others, and many which had different views from the others.

This is why I don't personally view these Indian texts as a "religious text", they're moreso spiritual knowledge documented by various masters, which a number of vedic/Hindu traditions happen to reference.

He was unsurpassed in his attainment and that's why we learn from him instead of the gods, who could've just been advanced ETs for all we know.

^{^}