You would need to write a Power Automate Flow that on creation of a new item in the site pages library set the permissions on that item to only allow the person creating the item to be able to edit.
I'm slightly hijacking the thread here but I'd be interested in more detail on this. I've tried to action this sort of thing within Power Automate before but found that I was unable to change item level permissions without a third party add-in. I think Serge Luca covered something in his blog last year that utilised a web service, but not everyone has development skills. I'd be interested in knowing how you managed this.
You can definitely do this, I helped build a flow a while ago that did this. There was some new functionality that came out at the time that made this easy to do. See here:-
