I worked in the music streaming industry, which involves ingesting 10's of millions of XML files. Absolutely do not use XSLT. Schema changes between different versions of DDEX were a nightmare for all the legacy XSLT we had.

XSLT and XQuery 3.0 can have type annotations and implementations can even type check based on a XSD schema. That should help with adapting to version changes in input and output formats.

I'm on the distributor side, I used to fantasize about using XSLT to produce all the different XML formats (beyond DDEX, like Apple, etc) from one house format, but that was probably a bad idea!