Your sworn testimony that you served the document is evidence.
Its actually literally the best evidence in our legal system.
Not sure why you keep bringing up the bailiff - they aren't going to serve these notices.
s85 of the Act covers the service methods of notices in WA. Delivering by hand is actually probably the best way, if you can be bothered.
That said, do you know what you need to do to best follow the eviction process?