There are several instances where the decision to use an item can't be automated.

For example:

• Using either a sword or a axe to fight mobs
• Using either a shovel or hoe on grass
• Using a silk touch pickaxe or a normal shovel on grass block

I suggest adding a keybind to use toolbelt instead of hotbar.
Like scrolling through the toolbelt and using 1-5 keys to select tool.
Should be simple to grasp.

Aren't current mechanics enough?

I have found myself multiple times moving tools outside of the toolbelt into my hotbar.
Like going on a stroll in the night.
Farming bamboo (why does it not use the sword automatically (b/c of the increase durability penalty??))
For me the toolbelt many times just becomes extra storage space for tools.

Also further changes for the toolbelt

Allowing multiple items to stack in a single toolbelt slot, that can be scrolled through by either long click or alt key while selecting the slot.