Is one month too long of a stay in Lucca? We are 77yo, plannng on mid-April arrival, our fifth visit to Italy. We won’t have a car. We would day trip to Milan, Florence, Cinque Terra, etc.
Depends on who “we” is, what time of year and where else you’d want to visit
Milan is too far as a day trip 3-3.5 hours
If you are planning to visit Tuscan hilltowns Lucca is really a bit too far north and west
Lucca does not have the best transportation links either
Lucca's a great choice, IMO. It gives you more stuff to keep your interest and, depending upon the season, there's wine festivals, comic/toy fairs, etc. Good choice of restaurants, easy tourist experience, music & arts.
I'll mention the only "downsides" (that's maybe not even the right way to put it)
Lucca doesn't have the greatest rail connections but it's not terrible. You can always rent a car for a few days or just deal with connections.
The historic center where you should be staying, is really a tourist destination. So you'll see busloads of day trippers and interact with a lot of tourists. The vibe chills at night, but it's still really a tourist area. Don't get me wrong; real Italian families live inside the walls, and the further away from Piazza S. Michele or Piazza dell' Anfitheatro, the more locals you'll interact with. And there's a whole work-a-day Italian city wrapped around the Centro storico
If you are into long range day tripping, Florence is the only choice, as it is linked by fast trains (250 km/h) to a lot of places. You could daytrip to Milan, Bologna, Venezia, Rome and even Naples is not impossible. Maybe it does not make sense to stay six hours in Venice but technically is possible.
Lucca simply has not such links, to get in any place you have to go to Florence first and you are already 1.30 hours travelling in the morning and 1.30 in the evening. Lucca is a nice place to be, but do not plan travelling very far from there.
Years ago I spent a week in Lucca. It was lovely. Especially in the mornings and evenings. I did 2 day trips to Florence and a day trip to Cinque Terre. Unless you have a "plan" on how to spend your days, 1 month may be a bit too long.
For me, Milan would not be a day trip. Much too long of travel time. Fly in or out of Milan and spend a few days in Milan.
I would limit day trips to maximum 2½ hours travel time in each direction. And only a very few trips that long. Anything further, I would overnight.
Here's a post from last year discussing day trips.
Unfortunately, April is not opera season in Lucca. However, there are usually concerts in the evenings.