Approval workflow - How to delegate approvals and capture who approved?



I am building an approval workflow in Smartsheet to track PTO requests. The workflow is built to send approvals to the employee's manager.

What I am trying to do is build a process to delegate approval when managers are on PTO for example and track who approved.

I was initially thinking of updating the "Reports to" column in the EE roster (separate sheet). But if I do that, I will update for past requests too.

I tried to use the "change cell value" automation to update a column "Approved by" (not represented in the screenshot), but you cannot only default the value with a dropdown menu. That won't work.

I also tried to use the system generated field "modified by" to capture who approved, but with a workflow, the value stored is not a user (assuming this is because the approver does not log into smartsheet).

Any thoughts to workaround the system and capture who approved a request in an approval workflow?