My elderly parents have had luck with big brand travel operators that operate buses and schlep luggage from hotel lobby to hotel lobby.
Search dcurbanmom Venice hotel ground level on regular Google to find a recent thread about hotels of interest.
In Florence, we liked Rick Steves' indie quirk reco - Hotel Torre Guelfa. It has an elevator to most room levels. If they are able to do some narrow steep stairs, there is a rooftop view level.
I have not used this company but it literally has been around for decades so I assume they have a good product. Their site says to expect 10-12,000 steps per day. I looked into their tours for a trip I ended up delaying and by then I was confident enough to fully plan on my own.
https://www.perillotours.com/italy/escorted-tours