Salesforce Formula Text Value Based on Custom Field Picklist Value

Question

on Cases, I have:

Department(picklist values=Ecom, B2B, etc.) Manager(text field) Employee 1(text field) Employee 2(text field) Employee 3(text field) Employee 4(text field)

This is what I am trying to do in real time, not on save (workflow rule won't work because it only does field updates after creating the ticket, not during the creation of it): IF(ISPICKVAL(Department__c, "Ecom"),Employee_1__c="John Smith", Employee_2__c="Jane Doe", etc.)

It keeps telling me I can't use a formula on the picklist field so then I tried to do it on each of the employee fields but they are not formula and only text fields.

I created another field that is Formula(text) and put in a formula:

IF(ISPICKVAL(Department__c,"Ecom"),"test",null)

This works but only for the formula(text) field, if I try to have it update the other text fields, it also does not allow me to do this and gives a boolean error, tried replacing null with "false" "true" with no difference.