If you are going to spend 8 days in Amsterdam then you need to think about it more as visit to Holland. The dutch rail network is second to none so get out and about

I liked going between Venice and Paris on an overnight train, in a 2-person sleeping compartment.

I agree with others that your division of time between the cities seems odd, but guess you have your reasons.

If taking the Artesia night train between Paris and Italy be sure to 1- book as early as possible as early but limited fares can be much cheaper than those bought in Venice - and if you have a pass book this train with the passholder rate (you pay the sleeping accommodation fee but the pass pays the actual train fare) make that booking with the place you buy your pass from - these trains can be popular.

And 2 - if a couple pay extra for the private 2-person compartment and not less for the 4- or 6-person compartments where you will be with strangers, often of mixed sex, etc. That is if you have never ridden an overnight train before - if you have and are OK with the 4- or 6-person compartments then fine - if not go for the double where you can lock your own door from the inside and out and there should be no problem with theft - others snoring, getting up at night, leaving the light on, etc.