URGENT! implementing simple document management system workflow for official letters in company

Hello

at first, I should apologize for my English!

we have a website up and running and we want to implement a very simple document management system for our office letters. this is done in the back-office and for employees. the scenario is:

user 1 (like the manager, boss, etc) writes a letter which may contain some tasks or information and submits that to user 2. user 2 may accept and close the letter or send that to another user (user 3,4, etc) and for every stage, they can make comments on that letter. every user has something like an inbox which contains letters that submitted to them and for every new letter they will be notified by email or something else. this is the workflow.

I can implement all the stages with views, rules, content type and so on. but the submitting system is the real issue. I wanted to implement that with the workflow module but it lacks some features. for example, it cannot deal with users. for my project the "workflow states" are the "users" and transitions are "changing the state from one user to another user". workflow does not have some automation tools to implement that and if I want to implement that in workflow module I need to define every stage manually for every user and shortly my workflow system is messed up!

so do you know what solution is right for me?