I'll try to describe my idea as clear as possible.

So, the first thing I suggest, is a simple pouch progression. it's unfair when u can upgrade your belt to the final stage just after u crafted it, IMHO. All pouches may have the same texture, but different meta, and each has its own recipe, from primary to the hardest to craft. So, each pouch has own slot it can add to the belt. For example, first pouch adds 3th slot, second - 4th slot, third - 5th slot etc.
(I've just came up with all recipes and properties, so if u're interested in it or don't wanna waste ur time on such things, just contact me).

And the second feature I suggest is belt durability. I don't understand the reason of thing, that u craft once, and then even ever put off it from its slot. So, every time a player insert or eject item from belt, it takes damage. If toolbelt takes a "fatal" damage, it doesn't break, just player can't open it via hotkey until he repairs it. (ToolBelt still can be accessed with toolbelt inventory)

And the last feature, that is connected with previous one, is the belt progression.
Player can upgrade his belt by putting it in a crafting grid and surrounding it with certain materials.
So, Player can't craft second toolbelt without crafting first one etc.
All pouches are on their places when belt is upgraded, so played doesn't need to make them all again.

• So, the first toolbelt is your original toolbelt and has 500 durability, and it can be repaired with leather.
• The second belt is iron belt and has 1000 durability (and 25% chance to avoid damage, optionally). Can be repaired with iron
• And the third belt is diamond belt and it has 1500 durability (and 50% chance to avoid damage, also optionally). Can be repaired with diamond.
  For each belt, it can be fully repaired with 2 its own repair materials.

Sorry, if u don't like such detailed suggestions.
And if u will implement all this things, please, make it for 1.12.2 too, not only for newest versions such as 1.15 etc.

The similar durability feature has mod MultiBags. There, whenever u either open or close (I don't remember exactly when) the bag, it costs 1 durability of the bag.
But player would open his toolbelt more often, so belt should have enough durability
My numbers above may be too high.
So, first belt may have 400 or 350 durability
second - 800 or 700
third - 1200 or 1050
P.S. It should be better to make it configurable via config))