Venice to ravenna

Welcome!

I think it’s a bit far for a day trip
Train to Bologna or Ferrara then transfer to a Regional will take 2.5-3 hours depending on connections
You can look it up yourself at Trenitalia
https://www.trenitalia.com/content/tcom/en.html

We spent 2 nights in Ravenna and were able to see all mosaics in town

Much as I love Ravenna, I'd probably make a different choice. It's not just that you'd face a lot of train time, but also that you wouldn't have enough time to see as much as many people would want to see in Ravenna. One of the top mosaic sites (Sant'Appolinare in Classe) is outside Ravenna and takes a bit of time to reach.

Closer options include Padua/Padova (26 minutes and up via cheap regional train; pre-book Scrovegni Chapel), Treviso (31 minutes and up, also by cheap regional train) and Vicenza (46 minutes and up via regional train).

I wouldn't do it.

We visited Ravanna a year ago, in October.

Day 1 - We flew into Bologna and took a train to Ravenna.
Day 2 - It's a small enough city that we covered almost all the UNESCO sites in the next day.
Day 3 - We caught our last site on the way to the train station.

Is it 2.75 hours for the fastest trains? Each way? Plus, you're at their mercy - I've missed trains and I've been there when there was a bus strike.
It's about 2 hours each way if you drive.

Ditto, we came to the conclusion that Ravenna was too far from Venice or from , Padua, for that matter. Another option from Venice is Verona. I was surprised with Verona. Because, you know, Romeo and Juliet. Yet we really enjoyed a lot in Verona, from the churches to the roman ruins.

Murano and Burano and Torcello (by boat)
Chioggia (bus)
Padua
Treviso
Vicenza
Verona
Ferrara
Bassano del Grappa
All of the above are within 75 min from Venezia.
Ravenna is a long 3 hour trek.