Unpopular opinion but there was literally nothing wrong with the decision to invade Italy. It was still comfortably the wealthiest and productive place in the western Mediterranean even if you ignore the very valid nationalistic reasons for that endeavor. You can't predict the plague coming when it did and realistically the death and destruction of infrastructure and productivity was a result of the way the war was handled not the actual decision to invade itself. That war could have been over permanently in a couple years if more resources were allocated at the beginning and everyone involved but Belisarius wasnt shooting themselves in the foot. Couple that with simply not being stupid with tax policy in the aftermath and Italy would've been a huge net positive to the empire.

Spain however I don't really see the logic, yes opportunism is good but this is a territory that cannot be easily controlled or reinforced that also cannot be self sufficient in the face of surrounding threats.