We enacted term limits for Presidents for the simple fact that a popular president is very hard to get out of office and thus they become very powerful. I have no doubt that Raegan would have made a 3rd and possibly a 4th term if he was allowed. Recall that by the mid-90's he was diagnosed with Alzheimer's, and there is much talk of him showing signs when he was still in office. I clearly remember the amount of age jokes that were said about Raegan and he was much younger than either Trump or Biden, or at least a bit younger.
The fact is the longer they stay the more powerful they become.
I'd put the limit at 12 years for congressmen (6 terms) and 18 for senators ( 3 terms). For congressmen who become senators I'd limit them to 24 years total in either/both houses.