Rome or Venice First after Cinque Terre (Monterosso)

You can fly very inexpensively within Europe and it will save you time and energy! Rome FCO has a very nice terminal. There is another airport used mostly by RyanAir nearer to the city, Ciampino.
Use trains within Italy.

I don’t think I’ve heard too many people call a night train a nice experience. I would absolutely fly.

As for the order of cities, I would base it on easiest/quickest train/plane schedules. You’re really cris crossing the country. I would look at which itinerary would waste the least amount of time. You might find that it makes more sense to go:

Interlaken
Venice
Monterosso
Rome
Munich

Rome’s airport is outside the city, easily accessible by train in about 30 minutes from the Termini train station.

Yeah it seems this new route you suggested saves me a lot of time going from Italy to Munich especially by flying. Now to play it safe I'm just going to recalculate my original itinerary from Venice to Munich by flight to compare apples to apples.

I read flying should be considered if the train ride is over 5 hours. Should I also fly from Interlaken (via train ride to Bern&Zurich) to Venice? I checked rome2rio, both train options go through Spiez then one more connection in either Milan or Brig. Never even heard of the latter. I feel like Milan is safer bet if I decide to take a train, since its bigger?

I LOVE overnight trains! But I also only consider private sleeper accommodation, never a bunk in a shared couchette. This can make the experience harder to justify vs flying.