The most realistic way to get from Paris to Venice by train is to take the train from Paris to Milan and then from Milan to Venice. Fortunately the fast trains from Paris to Milan are running again - taking about 7-8 hours - and there are many Milan to Venice trains which take about 2.5 hours. So you're looking at about 10 - 12 hours to make this trip but it is possible.
If you want two full days in Venice that means spending 3 nights which is the minimum I'd recommend to explore Venice - especially because you won't be getting into the city before 5:00pm or so your first night.
Fast trains easily connect Florence, Rome and Naples so that is pretty straightforward but you will need to expand further on what you mean by "Tuscany". Florence in is Tuscany but if you mean the rural, hilltown Tuscany of films you find these places are generally not well served by mass transit. You can either rent a car or take organized tours from either Florence or Siena to explore these areas. Siena is well served by trains and buses and is often many people's first taste of "Tuscany" for that reason.
There are two train companies in Italy - the state run Trenitalia and the private run .Italo that only runs fast trains between big cities.
https://www.trenitalia.com/en.html (Use Italian names for cities - Firenze for Florence and Roma for Rome - for best results)
https://www.italotreno.com/en
Fast trains have reserved seating, multiple classes and you can buy tickets ahead of time at a (sometimes significant) discount trading flexibility for price. Both companies have apps that let you buy and store tickets as well check trains in motion. I recommend getting at least the Trenitalia one if not both. You can generally only buy fast train tickets about 2 months ahead of time so assuming you're traveling until later in the year you can put off worrying about booking those tickets until you're closer to leaving.
The Man in Seat 61 is a great resource for looking into European trains and train travel.
https://www.seat61.com/Italy.htm
Hope that helps, have a great trip,
=Tod