Yes the grid must be upgraded, not only for transferring energy across countries, but also for electrification of heating, mobility.

Fossil fuels aren't a solution, and nuclear energy only a smaller part of it. You can see that if you look at the cost and potential in the IPCC report.

There is no way around a massive build out of renewable energy.

PS: I just checked energy charts, and saw that Poland has more wind in the winter as well