Train Reservation Needed Venice to Geneva?

You didn't mention it, but sounds like you are using a rail pass. If you were just buying tickets, the reservation would be included with in the price.
If you are using an EC train from Milan to Geneva, you still need a reservation. Otherwise, you would need to make several connections on local trains from Milan to Geneva.
You could make it with a connection in Milan and pay the $13 each for the Venice-Milan and Milan-Geneva legs.

As Sam mentioned, could you clarify if you're using a Railpass?

The Venezia S. Lucia to Milano Centrale segment will definitely need a reservation, which will be specific to train, date and departure time. It appears that you're planning to take the 08:01 departure, as that seems to be the only train with one change in Milan.

The price you quoted for the reservation on the Milano Centrale to Geneva segment seems a bit high. The Bahn.de website doesn't specifically state that a reservation is required, however the EuroCity trains usually do require a reservation so it would be a good idea to have one. If the Conductor checks your ticket / pass in Italy and you don't have a reservation, you could face hefty fines which will be collected on the spot.

Yes, we are using a Rail Pass, sorry I forgot to include that above. Thank you both for your help! We are feeling a little overwhelmed with everything for the trains so really appreciate your advice!

The rail pass you will have - is it a global pass or just a few countries? Is Switzerland included? Which route are you considering for Milan - Geneva?

A really good place to go for almost any question about trains in Europe with prices and recommendations for which trains are best for a given situation is this site:

The Man in Seat 61

https://www.seat61.com/index.html

It's easy to use and actually very accurate given the huge amount of information handled.

Thank you for the link!

We have a 4 country rail pass that includes Switzerland.