I like the 6 term House, 4 term Senate, 2 term president, and 20 year SCOTUS limits. I would also put age limits in place: no person can hold office if they will be 80 years or older at the end of such a term.
When the Constitution was created, it was extraordinarily rare for men or women to live beyond the age of 80 so there was no reason to add it. Now it's fairly routine, though physical health and stamina frequently decline rapidly thereafter, while the chance for debilitating diseases - cancers, strokes, dementia, and so forth, increases dramatically. I like formalising the concept of an Emeritus role (formally an Elder Statesman role) that would give people in that role exceptional access (Tribal Elders) but no formal authority.
Yes, you can point to people who have been powerful politically into their 80s and 90s (Pelosi, Biden, Ruth Baders Ginsberg, etc.) but it would also reduce the problem with people in decision making roles that shouldn't have been (Reagan, Trump, Strom Thurmond, etc. ).
I'd also be open to a 3 term president (present incumbent not withstanding). Twelve years is actually a pretty good arc - it gives you long enough in office that if you are an effective president, you will accomplish the changes that you want without becoming a permanent institution, and it mirrors that of other representative democracies. Yes, you will occasionally get the Trumps (though an age limit would solve that problem) and it would also have allowed a popular president such as Obama the chance to cement his legacy - imagine what Obamacare would have been like if he had been allowed to shape its passage another four years.