It feels that while gerontocracy is a valid critique to illustrate the problem, it doesn't fully capture why our processes actively choose such bad leaders. Rather it kind of papers over the problem assuming that mental faculties mean good policies [0], give us a comforting thought that most of the recent relevant candidates would have been out of the picture, while not actually addressing the "shit sandwich vs turd torta" dynamic.
As for the Senate itself, I'd propose increasing the number of senators to 6 per state to dilute this effect of a strong senator being a boon to state regardless of how bad their politics are. Perhaps straight term limits for the Senate and the House as well.
[0] while it certainly means better policies than what we have now, the bar is currently on the floor.