The interesting middle ground might be to prohibit anyone from serving more than two contiguous terms in the Senate or four in the House. Then if you've done your two terms in the Senate, you can run for a House seat, do three terms there and then your old Senate seat is back up for reelection. Except your old Senate seat now has a new incumbent who is only on their first term and you're running as the challenger. Meanwhile there are more seats in the House than the Senate, so if you hit your limit in the House you could go work for an administrative agency or run for a state-level office for two years and then come back, but then you're the challenger again.

The result is that you can stay as long as people keep voting you back in, but you lose the incumbency advantage and end up with a higher turnover rate without ending up with a 100% turnover rate. And you make them learn how other parts of the government work. It wouldn't hurt a bit to see long-term members of Congress do a two-year stint in an administrative agency once in a while.