Is it possible in SharePoint Online to change the "Add a page" link (from the Cog menu) to create a new Wiki page instead of a Site page? I'm sure our instance used to do this.
If you go to your
Site Contents / Site Pages and click on the 'Files' tab, 'New document' + down arrow in the 'Files' ribbon,
you are given the choice of the three different page content types, with 'Site Page' as the 1st choice, rather than wiki page.
It's also good practice for front-end users to go via their Site Pages library so they can check which pages have already been created, before they create a new one.