First time going to Europe-Itinerary Ideas

With 30 days you have enough time to visit your listed cities.

Vienna would be a good stop between Venice and Prague. Although Venice to Vienna is already 7h40 (there is also nighttrain running on that route). From my experience 6 or 7h during the day on the same train is not that enjoyable. From Prague you could then travel via Germany to Belgium or to the Netherland. I would recommend doing Prague to Dresden. And then visiting the places that interest you in Germany. 

If you only Plan to do Paris in France then i would recommend starting in Venice and doing France, beliufm and the netherlands at the end.

Otherwis starting from Paris and doing france first before heading to italy seems like a better plan.

Advice:

• dont take too much luggage with you
• Avoid the last train of the day if your journey involves connections
• long trips are more enjoyable if you make a stop for a couple hours in a city to break it up. 
• If you travel alone getting optional reservation for trains does not make sense (may-june should be fine in that regard)
• In places were reservations are not mandatory on the faster trains (Austria, Germany, Switzerland czechia ) you can be quite flexible

You should definitely include some stops in smaller/lesser known cities. You’ll have a more authentic experience and accommodation is cheaper too. 

In Italia for example Trieste, Ravenna, Urbino or Bologna. 
in Germany Trier, Bamberg, Lübeck, Freiburg

But there are so many more, and you’ll get tips here for sure, if you get more specific, where you want to go
 

There are some suggested itineraries on the Interrail website.

https://www.interrail.eu/en/plan-your-trip/trip-ideas/recommended-routes

You can use these as a starting point, then adjust to suit your preferences.

On a long trip like yours, you’ll have plenty of time to visit plenty of destinations. I would strongly recommend planning a long stop - a week or so - in one location about halfway through your trip, to allow you to chill out, do some laundry, do a few day trips etc.

Thanks for the great advice and suggestions everybody.  Very loosely, my idea for a route for this trip is to start at moderate pace the first week at 4-5 days in France, not just Paris, but a few cities/towns that are close by.  

For my second week, I’m thinking a night train to Italy for about 6-7 days for Florence, Bologna, Venice.  If there is time, I may stop by Trieste and go to Ljubljana, Slovenia for 3 days.  

There is a suggestion stopping at Vienna before going to Prague.  I’m thinking about 3 days each.  Then maybe Germany for 3-5 days for Berlin and Cologne and maybe some of the towns somebody suggested.

My last week would be in the Netherlands in the Amsterdam area for about 7 days and if there is time make a day trip to Antwerp, Belgium.

Is this doable or am I over extending myself and spending too much time in commute?  I estimated about 40 hours in commute.

This souds pretty reasonable. (I have done trips that involved quite a bit more travel in 3 weeks)

What is you plan to get from France to Italy? (as far as i am aware there is no night train from france to italy) 

Florence, Bologna and Venice  all have short travel times inbetween 

Venice-Ljubjana: Depends on the connection you choose, as you dont have that many options to choose from ( I have also read that there will be no direct train anymore from trieste to Ljubjana)

Ljubjana - Vienna 6h

vienna - prague: 4h

Prague - Berlin: 4h

cologne - Amsterdam: 3h

There used to be night trains from Paris to Milan, but not anymore. 
But if you make your way to for example Strasbourg  you can go through Switzerland to Milano, with options for scenic detours and/or stopovers.

Or you take a sleeper to Nizza, explore the Côte d’Azur a bit (Antibes, Cannes, Grasse) and enter Italy in Liguria.