Single Stock For Product Variation

Hello everybody,
I have this problem:
- we sell LCD refurbish, for different models of mobile phones...
so i have 1 product, ex: LCD iPhone XR with a fixed price.

If the client want to bring the old LCD with broken lens, will make a almost 50% discount.
If the parts have more defects, we have an extra tax.

So regarding cs cart VARIATION:
- if i add to variants with different prices for each option, the stock is not 1 product, is taking from inventory and each variant have own stock (see pic. 1)

I try to chose OPTIONS, i have another problem... prices are with discount, for each options, but are same for each product... if i add same Option to a 2nd product, with another prices... will change to the 1st one also... (see pic.2)

If there is a way to have same product inventory for each variant...
Or if i can make 3 option that can have different prices regarding each product.

Thanks in advance!