SharePoint Task list help

Hi

I am trying to build the task list to track items that are expiring in 30 days and send alter to email.  Can anyone help?

seems like the timeline will show as the due date in the task list.

I created a column 30 days before expiry and it's a formula calculate due date - 30 

how set the column of the 30 days before expiry to show in the timeline instead of the due date ? and how to set the email flow to alert the assign persons?

I like to see the start column too in the list.  and how to attached file, there's a attachment column.

can someone help me on this.  Thank you

thank you