Sadly, with wobbly legs (mobility problems) I think a Nile cruise isn’t really possible. As you say, clambering around boats would be a problem - they moor alongside each other overnight and the more expensive boats get the outside, berths, so cabins have a river view - that means to disembark, you have to cross, via gangplanks from one boat to another. As I remember, there wasn’t a lift and meals were taken down at water level.
Temples, etc aren’t visible from the river and often involve long coach trips, leaving the boat before dawn and even longer walks in searing heat depending where you go. It’s 45 degrees at the moment!
Life on board the boats varies from boat to boat and I guess you only get what you pay for.
We had good food, including afternoon tea taken on deck and cocktails watching the sunset, then evening entertainment after dinner.
We had a wonderful time other than almost being hijacked in the middle of the desert! We took a hot air balloon flight over The Valley of the Kings, but while you could enjoy life on board, I think the excursions would be too strenuous for you.