Looks like I'm not alone with this sentiments.
They could've just had him slowly being op with his equipment, maybe high C tier or low B tier. That way he can still improve, not just in equipment but in his stats.
What I would've written is since he was relegated as pack mule, he didn't get experience for his skill. His skill is only good for finding good things. And after gaining exp from fighting in an E rank dungeon, he gains exp that boosts his skill. Now his skill gets an upgrade; everything he buys gets added buffs.
This way he gets stronger, his former teammates don't suddenly become weak, and he quickly becomes stronger than them since they have plateau'd.