Assigining permission sets by apex trigger
Requirement : There are some fields on the object , we need to grant permissions to edit  based on the picklist values.( i have written validations rules at first but didnt work)
Scenerio:I ve made all the fields of the object read only in Profile. then created permission set to the fields for edit permission. now based on the profile name I want to assign the same permission set to the user though the trigger.but it is not getting assigned. kindly assist what changes should be done?

Trigger Attached

trigger PSA_on_WO on Service_Order__c (before update)
    {
    if(Trigger.isBefore)
    {
      if(Trigger.isUpdate)
      {
          for ( Service_Order__c wo : trigger.new)
             {
                  id id1 = userinfo.getUserId();
                 id id2 = userinfo.getProfileId();
                String pf=[Select Id,Name from Profile where Id=:id2].Name;
              
              PermissionSetAssignment[] psa1 = new List<PermissionSetAssignment>();

              system.debug('profile ----> '+pf);
              system.debug('id1 ----> '+id1);
              system.debug('order status--->'+wo.Order_Status__c);
              system.debug('order type--->'+wo.Order_Type__c);
                  if(wo.Order_Status__c.equalsIgnoreCase('Draft') 
                           && wo.Order_Type__c.equalsIgnoreCase('FS'))
                 {
                       if(pf=='profile1' || pf == 'profile2' || pf=='profile3')
                    {
                         PermissionSetAssignment psa = new PermissionSetAssignment(PermissionSetId = '0PS3C0000004GLCWA2',AssigneeId = id1);
                 psa1.add(psa);        
                                   system.debug('permission set id----->' +psa.PermissionSetId);
                          system.debug('assignid id----->' +psa.AssigneeId);
                    }
                    else if(pf=='profile4')
                    {
                    PermissionSetAssignment psa = new PermissionSetAssignment(PermissionSetId = '0PS3C0000004GLW',AssigneeId = id1);
                     psa1.add(psa); 
                    }  
              
                   }
             insert psa1;
        }
    }
    }
}