the game is simple if you focus in what you can you do for example if you can manage only 1 or 2 character having more will only give you the ilusion that you dont progress at all because you dont have enough time to gear all of them.
But i must admit that the game could change and implement non upgradeable gears alredy for example:
upgradeable: weapon,soul,pet,glove,bracelet,badges,soulshield,talisman,heart
non upgradeable: ring,earring,belt,necklace
having at least 4 equipament pieces that you dont have to waste materials/gold on them seem pretty fair we dont used to haved glove,heart,talisman maybe is time to change the old ways