My wife and I are in our sixties, and have no desire to get into technical diving, but we love learning new things and would willingly take continuing education type courses if they were offered. "What to do after Master Diver" is a great question, and one that has been on my own mind for some time now. We have considered Fundies, but honestly, we are a little daunted. Frankly, I think there is a real missed opportunity for the rec training agencies.
Thank God for Scubaboard, where you can learn a lot from others. It was right HERE that I learned about great trim, back kicks, and Backplate/Wing systems. And so much more.
If I were the OP, I would try to figure out what I wanted to DO underwater. Do you want to spear fish, take pictures, maintain wellheads, explore wrecks, or count fish species? The answer to that question will lead you to the training you need.