As I read the suggestions and your wish list, I keep thinking Bologna is the answer. There are direct flights from London into the Bologna airport, and a very short and easy taxi ride right to your hotel (it was less than 20 € for us) or monorail to the train station (but more expensive than the taxi option.)
The town is historic, lovely, and a pleasure to walk around, thanks to the famous porticos—-covered sidewalks that keep you separate and safe from traffic. It is also flat.
The area right around Piazza Maggiore, the central piazza, offers upscale shopping and a street full of restaurant tables and food shops. A market street is nearby. Even the small side streets without porticos offer safe and pleasant walking, because Bologna has lowered the speed limit within the city to 30 kph (although the limit is controversial and could be reversed). On weekends and public holidays, the main street leading to Piazza Maggiore, and the main cross street, are closed to cars, and the streets are filled with pedestrians enjoying the traffic-free stroll. We were there on a Saturday evening and the passeggiata was the liveliest we have ever experienced.
https://www.bolognawelcome.com/en/other/other/t-days-every-weekend-and-public-holidays
The city is home to the oldest university in Europe, and is full of small and sometimes quirky museums. It is also a Mecca for “foodies”, with a reputation for the best food in Italy.
There are lots of easy daytrips from here because the train station is a hub where many lines intersect. Easy options by direct train include Ravenna (1 hour 17 minutes by fast train, or 1.5 hours by cheaper regional train), Verona, 52 minutes, and Venice, 1.5 hours. Firenze is 37 minutes, and Padova an hour.
Addressing your concern about a daytrip to Venice being “stressful”, that can be true, but there are ways to minimize that if you plan carefully and are willing to spend a bit of money. For starters, when you arrive by train, do not join the scrum of people trying to board the vaporetto (water bus); walk to the waiting water taxis and ask one to take you down the Grand Canal to Piazza San Marco. It will cost more than 4 vaporetto tickets, but not outrageously more, and will be worth it.
But when you investigate all there is to see and do in Bologna you may not want to leave. See these websites for photos and suggestions.
https://www.bordersofadventure.com/bologna-emilia-romagna-italy/
https://www.italia.it/en/emilia-romagna/bologna
https://www.bolognawelcome.com/en