Is it permissible to complete the missed fasts of Ramadan due to menstruation in winter, or must they be made up before the current Ramadan, given the difficulty of fasting in summer due to work?

Delaying the making up of missed fasts until after the next Ramadan is not permissible according to the majority of scholars. One must repent for this delay and feed a poor person for each day, unless the questioner was unaware of the impermissibility of the delay. One must hasten to make up the fasts before the next Ramadan, unless there is a legitimate excuse for the delay, such as illness or travel.