When, if ever, will you be able to lock down the specific day and time of the trains you want to take for the long-distance travel legs? At this point you would be able to snag tickets at nice, low fares for most if not all of your long (potentially costly) trips. But they would be non-changeable/non-refundable tickets. No sense in risking that until you're solid on the timing of each travel leg.
The more tickets you'd be ready to buy soon, the less likely a rail pass is to pay off. Remember that if a train requires reservations (and many of the trains you'd want to take do require them), you pay extra for those reservations if you're using a rail pass, whereas the ticket prices you find on the internet (like those I show below) have the reservation fees already included. And in France, there's a quota on reservations allowed for pass holders on each train. If you didn't make the reservation early, you might not be able to get on the train you wanted later, despite having a rail pass.
A pass day will cost you a lot more than a local train ticket, so you can probably ignore any trips other than those on which you will be changing hotels--except in Switzerland, where it may be darn expensive just to step on any train.
A definitive decision in your situation requires that you research the cost of each significant travel leg and compare the total for each combination of countries to what a pass would cost covering the appropriate period. Yes, it is work.
For your trip from Barcelona, where are you headed in France? Do you have to go all the way back to Paris?
Here are some quick numbers; remember that I don't have your actual travel dates, and fares can vary, depending on day of week, time of day, and how far in advance you purchase the tickets.
Paris - Lucerne: 118 euros (for late April) to 180 euros (for today). This may well be your most expensive ticket. If you stick to this itinerary, there's a lot of money to be saved by buying this ticket early.
Lucerne - Munich: 30 euros to 80 euros
Munich - Venice: 40 euros to 93 euros. (This route goes through Austria.)
Note: I wonder whether there would be an advantage to traveling Paris-Munich-Lucerne-Venice instead, and buying just a Swiss pass of some type. Don't know what that would do to your travel time.
You mention a lot of travel legs in Italy, but Italian trains generally are not terribly expensive. For example, I found Venice to Monterosso (in the Cinque Terre) ranging from 30 euros to 65 euros; it may go both lower and higher; I only spot-checked. Florence to Rome on the fast trains can be as little as 20 euros if bought well in advance, or 47 euros for the current day. This is why the common advice is not to buy a rail pass for Italy; prices just are not all that high.
Madrid to Barcelona can be as little as about 50 euros for a sane departure time in April-May or over 100 euros if bought at the last minute.
Barcelona-Paris prices range from 49 euros (far in advance) to 184 euros (traveling today). Since this is at the end of your trip, you will probably know soon what day you'll want to travel, allowing you to buy the ticket quickly if you decide you will not be getting a rail pass.
Check Renfe for Spanish ticket prices.
Check Trenitalia for Italian ticket prices. Use Italian city names: Venezia, Monterosso (or other Cinque Terre town, but it won't matter much), Firenze, Roma, and Muenchen for Munich.
Check SNCF for French ticket prices.
Check Deutsche Bahn for German ticket prices.
If you need internal Swiss prices, they are on the SBB website, but you must double them because SBB assumes you have bought the half-price card.