Correct verb form

Hello everybody,

could you help me with the correct verb form?

Situation: I left ABC company in the beginning of January. The sentence will be followed with other considerations regarding my experience in the ABC Company.

Is it correct the verb form on the following sentence:

"My collaboration with ABC finished at beginning of January."

Several options:

"My collaboration with ABC finished at the beginning of January."

"My collaboration with ABC finished in early January."

"I left ABC in early January."