Hi, quick update on my train travels from Venice to Interlaken, in case it helps anyone.
Venice to Milan: Using the Trenitalia app, we purchased business class tickets the day before departure. Cost for adult: $64, child: 32
From Milan to Interlaken:
We had purchased the Swiss Pass. But based in what I had read and as others above have noted , the Swiss Pass cant be used for journeys originating outside of Switzerland.
So we purchased a business ticket from Milan to Birg ($57).
At Birg, we changed to a SBB train to Spiez, which was covered by the Swiss Pass. It was a bit tight - only 3 min between our train arriving at Birg and the next one leaving to Spiez. But worked out just fine.
ps: we were using the SBB app, and had connected our Swiss Pass to it.
And from Spiez we went on to Interlaken/Lauterbrunne/Murren - all of which was covered by Swiss Pass.
And final note - we used Google maps - it even has train times and platforms. Super convenient.
Thanks everyone for the help in planning for our trip.