Putin choose winter because every country that ever fought the Russians failed because of the winter.Russia has all the gas and oil it needs to maintain a perpetual war machine and a nuclear arsenal to back it up if it ever fails.Russia are effectively undefeatable using military means.Also their people are more resilient to hardship than western societies so winter would prove harder on our soldiers and citizens because we are soft whereas they are hard muddafuddas.If we engage in war with Russia over Ukraine which is literally ran by neo nazis then Europe will be the first to fall and after that anything goes.
Have You ever heard of "Rasputitsa"? Russia is not only known for winter but also for Rasputitsa. In spring and autumn, it's impossible to move with heavy machinery through unpaved roads because of the rain and melting snow. Those heavy Russian tanks will be useless in such conditions so it's better to do offensive in winter when the ground is solid.
War can not be won only by machinery but with manpower. And to properly fed the huge Russian army they need money for the provision and salary of the soldier. We all know Russia's economic condition right now so it's totally suicidal for Russia to start a full-scale war.