Stopover for the day in town along train route from Venice to Florence

You could stay flexible. Trains rarely sell out in Italy: you could book the evening before, depending on whether you'll have had enough of Venice. You will pay a bit more, but it is not extortionate, and there are two operators to choose from: Trenitalia and Italo.
For example, a ticket from Venice to Bologna tomorrow morning - a peak travel day! - is just 35€ on Italo, and Bologna to Florence is another 30€. That's not bad at all, in my opinion.

This might be fun for you to look up online "3D Flyover Map of Bologna". It looks intriguing, but would take a lot of study to narrow down your selections to view in a short time. I suspect you could spend a week there. Since Florence is so crowded these days, I would just hit the highlights and add an extra day to another location. Venice is also very crowded, but if you haven't been there, spend at least one day.

Hi, what’s your whole trip itinerary, so we’re giving you ideas that would round out your other locations?

I don’t remember a baggage spot at the Ferrara station (but I wasn’t looking for one), and Padova seems like it was also sparse, if any. So, that might eliminate one of your choices.

Padova has the beautiful Scrovegni Chapel - you need reservations, and that is close to the train station. There are two piazzas back-to-back - Piazza Erbe & Piazza della Frutta. We ate there and also at Piazza dei Signori in the evening. There’s a handy tram in the city, also.

Ferrara has a castle in the city center and excellent food. We took a taxi from the train station to the center. The museum with Spina artifacts is interesting, but that may duplicate something you’re planning in a different city.

It is a 20-minute walk from the Bologna Centrale station to the main square, Piazza Maggiore, heading straight down Via dell’Indipendza. It could take you longer than that to make your way out of the station, from the deep high-speed train tracks to the left luggage depot, and then to the exit onto Piazza delle Medaglia d”Oro. Make sure you know which exit you want; it is not all that well signed.

https://www.bolognawelcome.com/en/other/other/the-kipoint-luggage-storage-en

But it is a pleasant walk, especially once the porticos start.

Ferrara would also be a good option; there is a nice pedestrian path that starts near the station and leads directly to the main square. The train station is much smaller, and has no left luggage storage, but there are several commercial services nearby.

I've been to, and really liked, Padua and Bologna. I haven't been to Ferrara yet, but it sounds worthwhile. In that area, however, there are a lot of other great possibilities, including Ravenna, Vicenza and Verona. Rather than trying to squeeze in half a day in one of those many places, I'd spend as much time as possible in Venice where you won't be losing time getting from the train station to the historic area in a new city. You can do a deep dive into the smaller cities on a future trip.

My itinerary is as follows:

I know many warn of jet lag, but I am not the usual traveler. For whatever reason, I am on such a high when I arrive to my destination that I am full of energy and continue like that throughout the duration of my travels. It hits me once I return home. So, I expect to be ready to take on Venice upon arrival.

2 nights in Venice (arriving on a 1pm flight) = 1 full day and a bit of time on arrival and departure days
4 nights in Florence (no day trips) 3 full days with some time on arrival day
4 nights in Naples (day trips to Pompeii, Herculaneum and possibly Procida) = 1 1/2 days for city of Naples and some time on arrival day
5 nights in Rome (one day trip from Rome) = 4 full days and some time on arrival day

Rather than trying to squeeze in half a day in one of those many places...spend as much time as possible in Venice...

Agree with acraven 100%. Venice is so unique that it really deserves as much time as you can allot to it.