The impact of the winter tyres is quite pronounced in EVs generally (it costs us c. 3 kWh/100 km on the Tesla) - but in this case, the roof box is going to do major damage to the range, along with the fact you'll be climbing some 1000m at least going from sea level to the Alps. So budget for at least a 20% loss in efficiency, if not more.
As far as the driving strategy goes, uncertainty about chargers being available would suggest to leave some extra charge to make it to the next if necessary. But ideally, you'd make several short stops when you're nearly 'empty' - that way you charge fastest and can drive a reasonable Autobahn speed in between.
The trustworthiness and speed of the Tesla Supercharger network is why we won't use the I-Pace coming winter from our drives to the mountains, yet...