@Krixus you can make an inventory exchange look like slots (meaning a grid layout with the same shortcut slot UI images as your slot inventory, same UI box background images) but AFAIK not the empty slots that the OP specified, so you will only see the shortcuts for occupied space in the grid.
However you can also open a slot inventory in the same menu as an inventory exchange as another menu part, side by side. So like I open my normal tabbed slot based inventory side by side with inventory exchange Shop, and have an invisible box no raycast as the other exchange box so it doesn't show on screen, lol kinda hacky probably not intended.
Maybe this has changed since I set that up tho.
I used a hidden combatant with slot inventory as stash / bank rather than exhange because of this, in order to show empty slots in stash.