Where to see between Venice and Florence?

Looks good to me. Whether you will prefer more time in Ferrara or in Bologna is something you will only know once you've seen both. For me, Bologna is larger but Ferrara is far nicer; however, most people like Bologna a lot more than I do.

I'm glad you're visiting Padova. If you want to see Verona, it's only a slight detour from your itinerary, so don't discount it.

As for other suggestions, look at Ravenna, for its wonderful mosaics and general nice vibe. But, looking quickly at the Bahn website for trains, it seems all trains from Ferrara to Ravenna require going through Bologna, which adds time due to the backtracking. If you choose to stay overnight in Bologna instead of Ferrara (seeing Ferrara on the 13th on the way to Bologna), you can take a day trip to Ravenna more easily.

From the train station, the center of Ferrara is about a 15 minute walk. The center of Bologna is a farther walk from the station; take either a bus or a taxi. For Ravenna, I taxied right to San Vitale, then worked my way back on foot through town to the station; this worked very well.

Padova is indeed a nice day trip from Venice. Vicenza is also a beauty, and unmissable if you're interested in Palladio. The Regionale Veloce only takes 45 mins, and the regular Regionale 1hr 15.
I'd rather stay in Bologna than in Ferrara. Bologna is a nice, lively city and, as noted above, much more practible as a base for worthwhile cities around. At a few minutes from the railway station you'll find functional hotels.

Thanks massively to you both for your suggestions!

Ravenna is an interesting idea, though I think we would have to lose a day or two in Rome to make that work, probably by staying in Bologna for three full days and making Ferrara and Ravenna each day trips. As we both love Rome more than any other city, this would be a major sacrifice!

Vicenza is one that I had in my back pocket for another day trip from Venice, as escaping Venice during the daily crush is nice (even in March). I'm playing it by ear, depending on how jet lagged we are. Coming from the West Coast, we find it takes a couple days for us to get our feet back under us. This is why we're going to Venice first: it makes for an excellent decompression zone...

Instead of day-tripping to Padua, see it on the way to Bologna. There's luggage storage at the train station. My vague memory is that the historic center of Ferrara is some distance from the train station. In Bologna, I stayed at a hotel that was about halfway between the station and the center, not more than 10 minutes walk to either, making Bologna a better base for day trips. It also has many more restaurants.

If I could only see Padua, Ferrara or Ravenna, I'd choose Ravenna hands-down.

Haven't been to Ferrara but have seen Padua, Vicenza and Ravenna. I'd happily return to any of them, but gor me, too, Ravenna is #1. It's a twofer: All the glorious mosaic sites plus the beautiful town architecture. And in a nice, compact package except for one mosaic site that requires a local bus (if you don't havea car).