it depends on what you want to do in Egypt.
I took my mother (barely a pensioner) to Cairo 2 years ago, she can walk short distances but has arthritis in her hip and leg and can't manage steps too well.
We had taxi pick us up from our hotel (Mena House Oberoi) and he drove us up to pyramids, paid for our tickets at the gate and he drove us around each pyramid, helping her out of the taxi to shuffle about and take photos, then she would go back inside the taxi and I'd take some more photos then back in taxi and onto the next one, not forgetting the Sphinx.
Obviously a 4/5 night break in Cairo is more expensive than 7 nights in Luxor.
Luxor is a great place to relax but obviously there are things you would want to see. There is a lot of walking around the temples and it's not all smoothly paved. I've been many times and I even I struggle with the high, crumbly pavements.
Last time my mum and I were in Luxor, we were on a Nile Cruise. She had to carefully cross the "gangplank" and then there were lots of steps up to the road where the tourbus was. Stepping up onto the tourbus strained me! You're right, the excursions sometimes had an early start and we trailed behind the rest of our group. She still enjoyed herself but was exhausted at end of each day.
I stayed at the Sheraton (but nt with mum) and you would need a room in the main building (3 lifts), not in the garden bungalows where you have to walk along pathways.
At back door of Sheraton there is a sloped path or two down to pool area and gardens.
Bar and restaurant and reception area is flat. There might be a small step up into the hotel entrance.
The Mercure Coralia (Etap) Hotel is a lovely hotel as suggested above it is mostly flat inside and has a few lifts and restaurant on 1st and ground floors (and bar).
Would your husband consider Cairo for a short break?
having the luxury of being driven around the pyramids and then lounging about the hotel and gardens.
There are a lot of steps up to the 1st floor of the Egyptian Museum, I don't know of any lift. My mum took 20 mins going up them. And nearly twice as long coming back down.
I would say Cairo might be more suited. Sure, it's a busy city but it's not somewhere you would want to be walking around in with the hustle and bustle, you would only need 4 full days there and be taxied around everywhere.
I think I'm talking myself into a holiday.
Good luck, I hope you work something out.