What's best practice in regards to this? At the moment, our OOTB implementation has the notify user in the form of a Add Reminder function. The problem with this is that it doesn't send the email immediately. How can I change the reminder to send immediately instead? Or is there a better way that we should be sending the closure notification?
This advice depends on what your closure process looks like. Add Reminder is the action used to generate emails. If you want the email to be sent automatically then it needs to be placed in the process after an action such as resolved. The process will drive the reminder to be sent with the contents you require. Depending on the polling time of Outbound Mail Services they will be sent every minute, 5mins etc.
Reminders will only send if they are active (there is a tick box on the window)