What you're describing is basically Symphony, the CMS built around XSLT with some PHP and MySQL to glue things together: https://github.com/symphonycms/symphonycms

I don't think it's been updated since 2019. XSL was really powerful, but it had a steep learning curve, and I think server-side PHP and client-side JS were just more intuitive.

Cool project. Sadly it seems to be unmaintained