Tuscan or Umbrian Town Recommendation

Pienza is flat, lots of good restaurants

Buonconvento is flat but not much there

Orvieto is mostly flat in the center/top-easy to manage if mobility challenged

Both Pienza and Orvieto are quiet in morning and evenings once day trippers have left

The small city of Lucca is nice and flat, but going in the off-season and avoiding its big festivals are good ideas. For 2 weeks, you might want a small city with a pleasantly compact walkable historic center instead of a town. More to see and do, more choices of restaurants and coffee bars and gelato shops.

I enjoy this woman's blog about living in Lucca: https://www.twopartsitaly.com/

Bevagna is flat and well located for day trips.

I've spent some time in Lucca and it fits the bill but it is firmly on the tourist map, but nothing like Siena yet. It has the historic center but is a focus for the 80-90K people that live in the area so it has more than its share of good restaurants and shopping. It is light on sites - aside from many churches - but is an 1 hourish away from Florence and 30 minutes from Pisa by train. As for local lore there is a ton - between the patron saint of the town lying visible in one of the churches to the cross carved by Nicodemus himself that magically came to Lucca and its teleportation is recreated every year in a solemn candle lit passage.

I would aim to be there on shoulder seasons if you want a more Italian experience. Also check the weather because August and early September can be sweltering. There are several non-AirBNB sites that list longer apartment rentals if you end up looking seriously at staying there.

If you really want to get off the tourist track there are many cities that might fit the bill. You could look into Pistoia which is closer to Florence on the train and is often also considered by people who move to Lucca. Prato also fist that bill but has harder times since the wool industry has declined and is somewhat less "on the up" than Pistoia. Grosseto another historic Tuscan that is firmly on my radar but I haven't made it there yet.

All that being said, I have to ask "How is your Italian?" English drops off quickly once you get outside of more touristed areas especially among the older population. Italians are vey open to working with you and I don't think you'll starve or be unable to buy a coffee. But the inability to grasp complex answers or more subtle differences could but a damper on your integration.

Hope that helps, have a great trip,
=Tod

Lucca is not too hilly, but much of Tuscany and Umbria is hilly.
Still, it is wonderful.

Siena is special as are Perugia and Assisi, but there are hills.

You haven’t mentioned what you plan to do for a couple of weeks. Taking day trips to quaint hill towns would be what most people would do, but that might be difficult for your wife.
Poggibonsi is a small, mostly flat city centrally located in Tuscany and is a transportation hub. It has some historic sites to see, and a vibe that is definitely untouristy.

I realized I should add that in Lucca most of the old architecture had ground floors that were tall enough for mounted horses. So while places say they are on the "first floor" that means there is a ground floor and then the first floor is what we would call the second floor. And because the ground floors are really tall there can often be 20 or 30 steps up to the "first" floor.

So when you are looking at booking places in Lucca be aware of how many steps it may be to the place you are staying.

Something to keep in mind,
=Tod