Was leniency in changing one's specialization the reason for the lack of improvement in one's professional future, and would it have been possible to change the specialization if this leniency had not occurred?

Allah's decree is absolute, and belief in it is one of the six pillars of faith. It means believing that everything that occurs happens with Allah's knowledge and preordainment, as Ubadah ibn al-Samit told his son: "You will not taste the reality of faith until you know that what befell you could not have missed you, and what missed you could not have befallen you." Therefore, be content with Allah's decree, for divine choice is better than a servant's choice for himself, as Allah Almighty said: "And perhaps you hate a thing and it is good for you; and perhaps you love a thing and it is bad for you. And Allah knows, while you know not," and "But perhaps you hate a thing and Allah makes therein much good."