Whose font did Yongzheng learn and formed his own style?

It should be Dong Qichang.

First of all, the forbidden cultural policy and wild grass in the Qing Dynasty were not welcomed by the government. Kangxi and Yongzheng loved Dong Qichang's calligraphy, and admired Zhao's calligraphy when they were in Qianlong.

Secondly, it is recorded in the Chronicle of Forests that calligraphy was changed to Zhao during the Qianlong period, which also means this.