I agree that renown needs work. I think the avenues of renown gain should change depending on clan level. Things that get you renown early on shouldn't necessarily work as well or at all as your clan gets more famous.
For example, having a perk that gets me renown for operating caravans is fine, but that renown-gain should taper off and peak out well before I hit clan level 5 or 6.
Achieving the higher clan ranks should probably have some special requirements in addition to the steady accumulation of renown. Some sort of special accomplishment that sets you apart from the average nobles.
Regarding decay, I don't necessarily think I should be able to lose clan rank 6 once I have it, but perhaps there should be an additional layer on top of that that is more like a leaderboard - only the most fearsome commanders, or the most wealthy nobles, would maintain the highest ranking among the top clans. This leaderboard is in constant flux. There would be perks to being the best, but it would also paint a bit of a target on you - other nobles would want to defeat you in battle, or sack your ridiculously wealthy city in order to take its spoils.
Otherwise, criticism like this, "I find everything too easy in this game, and this does not only apply to renown. It is also too easy to keep 100 relationship with everyone," are valid, depending on the philosophy of the game:
Is the end-game supposed to be a more discrete experience that we prepare for and beat, and then we start over with a new game? This is at least somewhat how the campaign feels like it's intended to be. If so, then things like reputations and renown are basically fine, and the real problem is in other aspects of late game that make it too much of a cat-herding, siege-exploiting, slog.
Or is end-game supposed to be a much longer, almost indefinite process that feels like a real kingdom simulator, with more vague goals like "successfully manage as powerful/wealthy/large of a kingdom as possible," with the game designed to become increasingly challenging as you grow in power, with real checks and balances and risks. This is probably what I would prefer. If Taleworlds is going for the kingdom-simulator-sandbox experience, then lots of work needs to be done to keep it interesting, challenging, and immersive (like the early game is).
(It is worth noting that there is a reset currently in game for all those notables you have 100 relations with - they eventually die. Like many late-game features, though, the dynasty system needs tuning - the timing is just too slow to really impact a typical game right now)