Hello Adrien.
It sounds to me like you need to make two different product systems.
One product system consists of one process:
Input: Plastics
Output: Plastic wastes that are incinerated
The other product system consists of two processes:
Process one:
Input: PET (if you want to compare to the other system that includes plastic), electricity/transport (what is necessary for sorting)
Output: Sorted plastic ( lets say 1 kg. This will be a homemade flow, used as reference flow)
Process two:
Input: 1 kg of sorted plastic (from previous process. Remember to choose the provider to be the last process.). Furthermore, if you want to include recycling you should include the necessary flows here. For recycling of PET, i recommend the process called something along the lines of Production of polyethylene teraphthalate, granulate, recycled. If you use the cutoff approach, this process describes the production necessary to produce one kg of recycled PET, without any impacts from the previous lifetime of the plastics.
Furthermore, important to note. If you are comparing the end of life for two products, you might want to include displacement of energy and material after end of life. This is not in accordance with cut-off modelling, but it is interesting nonetheless. You can include the following in the two product systems:
System 1 output: Electricity medium voltage (Displaced product), District heating, natural gas (Displaced product). The amount should be equal to the net energy gain stated in the description of the incineration process, multiplied by the amount of incinerated material.
System 2: output: Polyethylene terephthalete (displaced product). Equal to the amount produced, (minus a waste in the recycling, if there is one).
These displaced products will show you how much energy production is avoided due to incineration, and how much impact from production of virgin granulate is avoided from recycling of PET.
Good luck!