The Turkish state is generally considered laicist/secularist with somewhat stronger links to Islam still intact (Islamic authorities must be authorised by a Turkish religious office). Because of the link to Islam, Turkey does not know of institutions called ‘church’ which makes it virtually impossible for a Church to be recognised as legal.
And do not forget what our Syriac and Armenian brothers have had to suffer from under Turkish rule. Lots of animosities are going on between Muslims and Christians, as can be seen in the murder of a Bishop.
I do not want to judge Turkey, maybe at the end things turn out a lot better than I’ve portrayed them - and I do hope so. Just be aware of the historo-political situation.
Constantinople (Istanbul) is said to have a good number of Western and Eastern Catholic parishes.