When should you move on from a postdoc position?
"Discussion" isn't really how SE is supposed to work. However, I think the answer to your questions are relatively clear-cut (at least to me):


  you are offered a postdoc position in a second-tier research group. Should you consider taking it, in order to broaden your academic horizons?


Why? You have a broad horizon, you have seen three top institutions before even becoming a faculty member. Go if the new position offers you something you are currently missing (such as a nicer place to live), but I would certainly not move just for the sake of it.


  [you are offered a ] a faculty position at the second-tier institution?


If I like the place, I'll take this offer without a second thought. Expecting to become faculty directly at one of the five top institutions seems too much to ask for.


  a postdoc position at one of the first-tier institutions you've already had an association with?


Did you like it there better than in your current job? If yes, take the offer. If no, stay.


  a postdoc position at a first-tier institution that you haven't already had an association with?


More or less same answer as for the first bullet. You have seen a lot of places already, you do not need to change jobs just for the sake of it. If there is something you think the new institution will do better, then go for it. If not, stay. There is no life achievement for visiting each top institution in your field.