What do you mean "making" it require concentration?
It does allready. :-/
About implementation ...
From my limited knowledge of coding i dare to say that there ahould be little difference between hand using its own or its owner action or bonus actions ...
The problem here would be to explain this to new player.
I mean yes there are tooltips but still some pepple will probably be confused.
Agreed on initiative but that should aply on any and every companion without its own mind. In my opinion at least.
I like the idea with its own inventory too ...
I just wonder if carry weight should be the only atribute here ... i mean it seems weird to imagine Mage Hand floating around with 4 Leather Armors just bcs they "technicaly" weight less than 10 ... doesnt it?
I think it should also ne limited with single slot.