Automate to assign in multiple person column

Hi Everyone,

I’m currently structuring an IT Board with 70 members, & looking for a automation when the product lead assigned, it will automate another person column as a Reporting to (which is his line manager).

2nd, is there a way that i can lock a Planned timeline after the first entry?
In IT board structure, user will be inputting the task and provide their planner timeline.
But i want to ensure that it will be locked to avoid on changing it and in order track if the task is overdue or on time based on the planned and actual timeline.