Based on your post, I understand that the default page used to be home page in all site collection includes sub sites cannot be edited, even use site collection administrator account and SharePoint admin account. Is it right? If it is wrong, please correct me.
1.Check the default page whether has unique permissions.
2.Make sure the default page is checked out, published and approved.
3.Switch to the modern experience, then try to edit the default page.
4.Clear the browser cache or use another browser to edit the default page.
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