When creating a custom Resource Class, how can I change the default value of a property?

Question

I've created a custom Class inheriting from a resource.

How can I redefine the value a certain inherited property starts out with?
Better yet, how can I override the default of a property exported by the superclass?

I tried the following approaches:

@export property_name = 64
# Unexpected "Identifier" in class body.

@export var property_name = 64
# Member "property_name" redefined (original in native class 'TileSet')

@tool
class_name CustomTileSet
extends TileSet
func _init():
    if Engine.is_editor_hint():
        property_name = 64
# Does indeed set the value, but not the default (and might cause other problems)

set_meta("property_name", 64)
# modifies the wrong kind of metadata (Object Metadata instead of class member metadata)