Export list of users with unique permissions

Hello community,

The context:

We have a document library with 30 000 items. We broke inheritance and set up special permissions for some of them. Now we have about 30 items with special permissions that can be listed in Sharepoint Online. See the picture below. 

What we want:

We want to get list of user on each item now. How can we do that?

The output should look like this 

item1 - user 1
item1 - user 2
item1 - user 3
item1 - group 1
item1 - group 2
item 2 - user 1
item 2 - user 5
...

The idea

We will write powershell script

1. But can we identify items with special permissions the same way Sharepoint does it in the picture? How?
2. How to use the API correctly because we cannot list each item (30k) in Sharepoint for performace reasons right?

Related topics maybe?

List of Unique Permissions from Site Settings | Microsoft Community Hub mentioning StephenRice​