I think there's a happy middle ground that I'm shocked doesn't exist or is obscure as hell because I've never found it in the wild is a "default" schema that is compatible with JSON types and can be serialized the same way JSON does. Because whenever it comes up on the on-disk format seems pretty irrelevant people just want to have an xml.load to map to their language primitives in a sane way.
There's this kind of stuff but it's niche and more convention than actual schema.
I think there could be, but it's ugly. The XStream library wasn't horrid, but it basically provides a way to serialize/deserialize from the java object defs, which is still a schema of sorts.
You know <key name="blah">key value</key>. It just highlights the extra verbiage, and pushes people towards "just use json/yaml".
There's this kind of stuff but it's niche and more convention than actual schema.
https://www.xml.com/pub/a/2006/05/31/converting-between-xml-...
https://untangle.readthedocs.io/en/latest/
https://github.com/martinblech/xmltodict