What is the ruling on working in a job that requires writing down usury and meeting women without a mahram and mixing with unveiled women in the workplace? And is there a legal basis for preferring trade over being an employee?

Firstly: Work that is inherently forbidden is not permissible, and similarly, work that is forbidden due to its mixed environment or conditions that violate Sharia (definition) is also not permissible. It is permissible to work in a mixed environment if there is a legitimate Sharia interest in doing so, provided that necessary precautions are taken to avoid fitna (temptation/strife). If one cannot preserve their religion, then they must leave the job.

Secondly: The professions of the Companions were varied, including trade, agriculture, crafts such as blacksmithing and carpentry, and occupations related to the state and war spoils.

Thirdly: It is not authentically established that the Prophet (peace be upon him) preferred trade over manual labor. The purest earning is a man's work with his own hands and every righteous sale, as stated in the hadith (definition) of Rafi' bin Khadij, and no one has ever eaten a meal better than that which he eats from the work of his own hands, as stated in the hadith of Al-Miqdam. The best earnings vary depending on circumstances and individuals, and one should choose what suits them and what they excel at.