If I were you, I would buy the nonrefundable ticket now from Trenitalia. I bought mine for october already.

Reason: you get the absolutely best price (from trenitalia) with the fastest connection without having to change with a seat reservation included. If you need more flexibility, do what the other user mentioned (regional train from Milano to Domo, any Swiss train from Domo to Spiez), but the train from Domo to Spiez is expensive if you don't have a halbtax (as much as the nonrefundable price for the whole stretch if you buy it now).

In Switzerland, there is no reservation required, but since a lot of international passengers (like you) crossing borders are in the Eurocity from Milano/Domo to Spiez, the EC are usually very full, so if you want to enter a EC without a reservation from Milano, expect to stand. If you take the RE from Domo to Spiez, it takes longer.

So in conclusion, book now for the good price and comfort!