Redirect user to landing page after workflow is manually started?

how do you start that workflow?

if you built your own start workflow link you could append &Source=.... to the URL which would redirected you to a given page.

see these

Redirect After Manual Start Workflow Completes 

I start my workflow from a custom menu label at the top of the drop down menu on the list item.

I removed the default start page URL in Workflow Settings because the user does not want to click a Start button.

However, they would like to be directed to a confirmation page after starting the workflow, not the list page.

then you would need to customize that workflow menu entry in SPD.