Hi @bjoern__koth
Sending custom link tracking calls does not rely on document unload it is used to make sure that your data will be send to adobe servers if you're redirecting quickly from one page to another however when document unload is checked you won't get a response back from the server that's why you'll see a collect call with no payload in it.
It relies on send beacon API to asynchronously sends a small amount of data over HTTP to a web server.
check this article for more details
Also, it is incompatible with the render decision property I guess that's why it is being blurred out because guided event's automatically select the value for event type for send event action.