“What is the foundation of conservatism then?”
That’s harde to say, but just off the top of my head, and speaking very broadly, it’s hard to imagine what conservatives would be without the Western Tradition, the Wisdom of the Ancients, and the oft referred to Judeo/Christian Worldview, and all predate the U.S. Constitution.
The Constitution was created from a distinct philosophy that incorporated the "Age of Reason" with radical ideas about individual freedom and liberty. It incorporated morality yet still remained secular out of respect for varying religious views. It wasn't based on a "tradition" or "wisdom of the ancients". It was new, it had never before been attempted, it was genius. It formed a nation, and a government that was ruled by the people without the obvious drawbacks of direct democracy. It limited the powers of government to only those enumerated. It held that all other rights were retained by the states or by the people.
What else could you possibly want to conserve any more than the Constitution, the very foundation America? We have fallen far. The constitution is the only way back.