Advice on 'Carton rate' discount app to replace Pack variants

[TradeGear]

Hi, was hoping someone can offer some advice as tested over 15 apps so far with no luck. We currently have a number of products we offer a carton rate on. These have been set up as product variants in our store, however because our inventory management is on the individual item, I am forced to select 'inventory not tracked' on these variants, and we are continuously selling cartons of stock that we dont have. 

App is required to sell our products as a 'box' - instead of buying one item, you can buy it in a box of 12 at a discount %. The app needs to discount based on multiples - i.e if someone purchases 14 items, 12 would be at discounted box rate and the other 2 full price.

App needs to recognise the total amount of products in cart - i.e if someone adds 10 items to cart, then decides to just get the box and adds another 2 items, it should automatically recognise this and apply box rate.

The app needs to deal with variants, but in a way that means a box can only have one variant type. i.e a box of 18mm tape contains 48 individual rolls but a box of 24mm tape contains 36 individual rolls. Same product but different quantity breaks for the different variants. Then, customers shouldn't be able to select the variant type - a box of 18mm tape can only be 18mm rolls.

Lastly, app shouldn't use checkout discount codes to apply carton rate, as this conflicts with promo offers and our B2B app.

Any suggestions?!