Godot 4 example on how to toggle @export visibility and readonly.

export_inspector_toggle_example.gd

@tool
extends Node2D

@export var show_number: bool = false:
	set(value):
		show_number = value
		notify_property_list_changed()
		
@export var editable_number: bool = false:
	set(value):
		editable_number = value
		notify_property_list_changed()

@export var number: int = 0

func _validate_property(property: Dictionary) -> void:
	if !show_number:
		if property.name == "number" || property.name == "editable_number":
			property.usage ^= PROPERTY_USAGE_EDITOR
	elif !editable_number:
		if property.name == "number":
			property.usage |= PROPERTY_USAGE_READ_ONLY