With a 15 day- pass in two months, you’ll also be able to do your schedule and provide you with some buffer days.

Luggage is no problem on trains.

Seat reservations can be avoided of minimised, when taking regional trains or slower routes (with regional trains). On Bern-Milano, and Milan-Torino seat reservations can be avoided, by taking regional trains but I’m not well travelled in Italy, so others will give you better advice on that.

Optional reservations you should consider paying, on busy days. (Long) weekends, fridays, holidays. When travelling on ordinary weekdays on long distance trains with optional reservations, are not really necessary.

You can’t avoid reservations on the Eurostar to/from London. Book it as soon as possible, they can sell out! The rest of the trains is less urgent, and probably have quite good availability.

www.raileurope.com is a good website to get most of your seat reservations (save for Poland and Czechia I think). Make sure to use the desktop versions, and select your rail pass in the search window below (“add rail pass”)

The weather will be rather pleasant, not too hot, not too cold, generally fair spring weather. But it’s the weather, it can’t be predicted so far in advance You won’t need warmer clothes, except in the mountains maybe, but you don’t plan to stay there I think.

You’ll surely have some nice spring days with above +20°c, especially in the south.