Renown problem should not be a problem, I think - we already have a system in place.
Hoard level only gives renown to 100, nothing past that matters in this respect.
Put the same hard cap on win-related renown so that it is only awarded up to delve 500 (like we have right now) and then one could go as far above as one desires and not exceed 2500 renown per delve.
In a certain sense, renown is a type of reward, so it is only logical that no new renown gets added if delves were to be extended past 500 as the OP suggests.
By the way, speaking of hoard upgrades being unrealistic: