Is it better to stay for an additional year in a non-Muslim country to obtain a master's degree, taking into consideration the aforementioned advantages and disadvantages, or to return after the bachelor's degree?

It is permissible to reside in non-Muslim countries under certain conditions: that the resident be a person of faith, knowledge, and insight, able to manifest their religious rituals, and secure for themselves and their family. If these conditions are met, there is no harm in remaining to complete one's studies. It is advisable to strengthen ties with the Muslim community in that country, and to be diligent in going to Islamic centers for prayers and attending lectures to strengthen faith. It is even better to participate in da'wah (inviting to Islam) and organizing lectures and lessons.