The question to ask is whether you could fulfill the contract obligations in Belgium. Moving to Turkey was a business decision.
An example to illustrate the contract obligations option as a transfer mechanism would be to use a Belgian travel agency. The data subject, hires the travel agency to plan a trip to Turkey (hotels, meals, site seeing). In order to fulfill the contract, it is necessary to transfer some personal data to Turkey (to the hotel, the restaurants, the site seeing company, etc..). There is no way to make my travel arrangements without sending the data to Turkey. You would be unable to fulfill the contract if you could not transfer data.
However, in the scenario posted, you could implement the infrastructure and resources necessary to fulfill the contract in Belgium. Because it was a choice to operate in Turkey, a controller can’t rely on the contract option because as a company the decision has been to operate in Turkey.