Imo,i never get why people blame justinian conquest for ERE overextension. People forget that belisarius conquered africa and 3/4th of italy with less then 25000 men.
Iberia might had been a mistake but still, Africa will later prove very useful during heraclius reign and italy serves as a shield to the imperial heartland of greece and anatolia, so its a net gain in the end. Its overextension was caused by the plague and If i am not mistaken, The Justinian plague will happen regardless of what he do.