This response may not be what you are looking for, but....
I was in your position when I was a few years younger than you. It was not just my age--I had severe osteoarthritis in my right knee, cause by a series of athletic injuries during my youth. I bought a pair of fins that were supposed to do what you want--Mares Volo Power. They worked as advertised. they were easy kicking fins that propelled me along nicely
Then two things happened at about the same time. I began working on technical diving, and I got the knee replaced. The Mares Volos were a problem for the tech diving, because they are designed for flutter kicking only, and technical diving requires a lot of different kicking techniques that are best performed by stiff blade fins. I bought the fins I needed for the tech diving and kept the Mares fins for open water recreational diving. Before long, I decided that I liked the fins I was using for technical diving better for all diving, and I sold the Mares fins.
I am now a decade older than you, and I own and use several pair of stiff blade fins. Because I have to carry around heavy gear for tech diving, I make sure my exercise routine keeps my legs strong. As a result, I have no trouble with that kind of fin, despite my age, and I plan to keep it that way for as long as I can.
So my advice is d
on't give in to age until you absolutely have to. If you decide that for the diving you will do both now and the future the easy kicking fins are just what you need, then that is what you want. But please make your choice based on the kind of fin you need for the kind of diving you will do, not because of an assumption that you are going to be losing leg strength soon. You are far too young to give in to old age! You want your legs to be as strong as you can make them for as long as you can, because you need leg strength to continue a suitable quality of life, whether that includes scuba or not.
Do not go gentle into that good night,
Old age should burn and rave at close of day;
Rage, rage against the dying of the light.
--Dylan Thomas