Train from Venice to Interlocken or St. Moritz

The "best" way is presumably the quickest. Rail websites always find you the quickest route. Go to the SBB (Swiss Railways) website: https://www.sbb.ch/en

Enter from: Venezia S. Lucia, to: Gimmelwald, and a date and time, then click on "Search". If you then click on one of the options, it will show all the trains involved.:
Train Venezia S. Lucia to Milano Centrale
Train Milano Centrale to Spiez (sometimes Brig)
Train Spiez to Interlaken Ost
Train Interlaken Ost to Lauterbrunnen
Bus to Stechelberg
Cable car to Gimmelwald.

NOTE: AFAIK Eurail passes are not valid past Interlaken Ost. you will need to buy separate tickets for that.
Seat Reservations are required for the Italian trains (Venice to Milan to Swiss border) if using a pass. If buying normal tickets, the tickets include all reservations. Seat reservations are not required in Switzerland.

"We would also like to see St. Moritz steam train." - not sure what train you mean, can you explain? But St. Moritz is nowhere near the route from Venice to Gimmelwald.

Nor is St Moritz anywhere close enough to Gimmelwald for a day trip. Suggest you go from Venice to St Moritz first if there is something there you want to see. Go Milano Centrale to Tirano then over the Bernina Pass by train to St Mortiz. Very scenic trip once you are 30 minutes north of Milan on the way to Tirano.

Edit - It takes about 7 1/2 hours to get from Venice to St Moritz and 6 to 7 hours to get from there to Gimmelwald. Its 7 to 8 hours from Venice to Gimmelwald. Those Alps are still quite a barrier.

I don't know of any steam trains in or near St Moritz.

Where are you looking that says that there is?

if your intended destination is Gimmelwald, St. Moritz is going to be a considerable (and possibly expensive) detour. Is the steam train you're considering one of these - https://www.rhb.ch/en/world-of-railway-experiences/steam-nostalgic-rides .

You will need seat reservations for the express Freccia or Intercity trains in Italy. Note that the reservations are specific to train, date an departure time, so you can only use the one train listed on the reservation. You'll be assigned a Coach No. (Carrozza) and Seat No's. (Posti). DON'T be caught without valid reservations for the train you're riding on, as you'll face hefty fines which will be collected on the spot!

The best for me (though not the quickest) is via the Bernina Express. We did your route in reverse in 2016. Venice-Milan by Trenitalia Frecciargento. Milan-Tiranno by Trenord regional service. Tiranno-St Moritz-Chur by Bernina Express. Between Chur and Interlaken we drove, but you can take SBB trains via Zurich or Lucerne.

The Bernina Express has panoramic or even open cars for some of the best Alpine views, a scary ride over the super high Landwasser Viaduct and the cool figure 8 Brusio Viaduct.

RhB does run some steam train excursions, but they are one day a month and at different locations. The one in St Moritz is Mother's Day, so that is coming up.