+1
I may not be an Ultimate, but I can understand how this will be very frustrating.
do you mean to smelt all of the items of the same type? or to smelt all items that can be smelt at once?
It could be like: /smelt all Cobblestone
or if you want to smelt everything that you can in your inventory you could have it:
/smelt all, then a prompt message will come up and you would say /smelt confirm.