Hi @Tamras,
You can use the when a file is created (properties only) trigger action and use the Site Pages library as a custom value in the Library Name dropdown. After that you can use a stop sharing an item or file and a grant access to an item or a folder actions to set the custom permissions.
Alternatively, you could also use Send an HTTP request to SharePoint actions for setting the permissions as well. That one is probably easier if you want to add a SharePoint group.
Below is an example (with the stop sharing and grant access type of actions)
