What are the oldest archaeological artifacts that prove Judaism existed thousands of years ago before the Middle Ages?

 None. The Ancient Canaanite religion is about 5000 years old and it was thoroughly polytheistic. Ba’al was an important deity. His cult included Carthage.

figurine of Ba'al

It’s not sure whether Ba’al and Yahweh were different deities.

El, the supreme Canaanite deity, father of Ba'al

The Torah uses the plural Elohim (the gods).

Yahweh, the storm god

This bull figurine could represent El, Ba'al, or Yahweh

In the twelfth century BCE the Israelites emerged; Yahweh became their national god when the kingdoms of Israel and Judah were founded around 930 BCE. Yahwism became separated from the Canaanite religion.

Around 600 BCE a number of Israelites went into exile into Babylon. Judaism originated among the exiles and the Torah was written. Yahweh and Elohim merged into “The Lord”. Over time Judaism developed from polytheism to monotheism, long after Pharaoh Akhnaten of Egypt introduced monotheism.

By that time, Rome had been founded. In 476 A.D. Rome fell and the Middle Ages started. So Judaism is a relatively young religion with ancient roots.