Tomasz - in this post (Task permission) it is said that setting individual permissions for specific task item will cause WF to stop working? For me this sounds like good idea:
You could however create a workflow on the task-list, which is triggerd when a new task is created. This workflow could assign individual permissions to the user to whom the task is assigned to. This way your main workflow would not be bothered with this "detail".
Will this work or not?
Cristopher in this post (How to hide buttons in SharePoint form ?) is suggesting hidding approve / reject buttons but this is not the solution suitable for enterprise.
Yes, setting permissions will work, however ensure, that you are giving the "Assigned To" the contribute permissions
Read my blog post about different approaches when handling tasks assignment and ensuring who can complete it or not:
Tomasz, in you blog to solve this problem you are suggesting creating WF in tasklist and setting permissions when the items are created. Creating this WF is easy but can you please elaborate how to trigger this WF since it cannot be triggered on task item creation (explained here: https://community.nintex.com/thread/8359). Also we cannot use Run workflow actions since WF is waiting for user action when the task is created.