I do not understand how you are conceiving of encumbrance as another layer on top of inventory slots instead of an alternative to it.

If encumbrance and inventory slots were layered atop each other, there should be a state where you have used up all available slots while still having remaining capacity in encumbrance: a thing that never happens. I have never seen a game with encumbrance reach this state except maybe Soulsbournes and that’s more about stat-gating equipment you can use than preventing players from carrying more things.