Suggestions for a Smaller Town in central/northern Italy - similar to Lucca

Vicenza
Padova or Verona for a larger town

All basically flat

Like Lucca, both Treviso and Ferrara are cities, but they don't feel like cities and are both an hour or less by direct train from Venice. We've spent a week in Treviso and almost a week total in Ferrara and found them very livable, walkable, with interesting neighborhoods and markets, not heavily touristed, and FLAT. Good food, too. Lucca was darn full of tourists last November.

Ferrara (just the cathedral and museum) --- https://iviaggidiraffaella.blogspot.com/2017/06/

Ferrara --- Palazzo Schifanoia frescoes cover the walls in a huge room and were fantastic in every sense of the word: https://smarthistory.org/sala-dei-mesi-at-palazzo-schifanoia/ Even a person with no interest in art would like to see these frescoes.

Ferrara --- Casa Romei had some beautiful frescoes in a 14th century merchant's house.

Treviso --- It's lovely, with canals and bridges, plenty of great places to eat, Venetian-style buildings frescoed on their exteriors, old city walls, several small churches and museums to explore, a fish market on an island, some cool wire mesh art installations on some of the walls of the buildings, and many pedestrian-only streets and porticoed sidewalks. If there were tourists there, they were Italian.

Treviso --- Here's some of the wire mesh art of Mario Martinelli in Treviso: https://www.mariomartinelli.it/en/ombre-rete/

If you are interested in smaller towns near Venice (the ones suggested above are small cities), consider also Montagnana and Cittadella. Both can be reached by train from Venice. While in Venice you might want to also visit Murano and Burano, if not already in your plans.

For larger towns (small cities) all of the ones suggested above would work. They are all flat and not much bigger than Lucca.

Senigallia would be another option--fly home out of Ancona?

Ravenna is worth a day.
In Tuscany, Siena, San Giminancio, Voltera and Pisa.