Perhaps you might say something along the lines of that you're reached a point in your life where you've achieved and fulfilled career and professional goals and are looking to focus on different areas going forward.
I think I know what you're trying to convey but it's difficult to get that across without, as you say, selling yourself short or sending up unnecessary "red flags." I would presume the cognitive decline you mention would not affect your ability in the sort of jobs you may be considering now. I think there's a way to state that you're still capable (and employed) at your current vocation, but looking to take things a bit easier — without using those words.
Or it may be as simple as to say that in those terms — something along the lines of "I'm an intelligent, accomplished person. I've enjoyed my time working at XYZ, but while I still want a fulfilling job, at this point in my life, I've earned something a little less intense." (Or a better word than intense.)