Is my supplication, "O Allah, grant me good in this matter," considered an infringement upon the Istikhara supplication, especially since I supplicated regarding a permissible matter?

There is no transgression in supplication in this wording. Nevertheless, it is better to entrust the matter to Allah and not to ask Him for something specific whose outcome is unknown. Rather, one should ask for the facilitation of good and supplicate with the comprehensive supplications established from the Prophet, peace and blessings be upon him. Ibn al-Qayyim warned against asking for something specific whose goodness and outcome are unknown, and he advised making the request conditional on Allah's knowledge of what is best in it, and to prioritize true Istikhara (guidance-seeking prayer).