My wife and I just finished a 25-28 Apr trip on the Legend and thought it was mostly great. We were on the N Central route with stops at Genovesa, Santa Fe, and multiple visits to Santa Cruz. #4's response about staff etc generally was our opinion also except we weren't bothered by the quick and easy life vest changes. We were fortunate to have good weather, calm seas, and the food was good and plentiful.
Re your husbands limited mobility. You may have to be more specific plus it would also depend the length and routing of your visit. You will see very little from the legend itself which means you will have to transfer from the Legend to the zodiac boats for transfer to the islands and their sights. Though awkward, there are plenty of staff there that assist everyone during this process. Then there are the landings on the islands. Most on our routing were dry but a couple were wet ie you had to shimmy over the side into the water and of course the reverse when going back. Of the islands/sights we visited, most trails were easily navigated and relatively level. The exception was Prince Philips Steps on Genovesa sland which meant climbing ~100 meters up and over uneven rocks. Again, what is your definition of limited mobility?
As far as activities go, there are activities scheduled throughout the day from 8-5 including 2-3 island excursions. glass bottom boat trips or snorkeling/deep water dives etc. Except for Santa Fe Island Barrington Bay, the glass bottom boat trips were disappointing due to reflections.
We had a lady in our group who had limited mobility who was having knee replacement surgery 10 days after leaving the islands, The guides (and rest of travelers) were very helpful to her and we never felt that she held us up. Hopefully they will do the same for your husband and you both have a memorable 40th anniversary.