I don't fully understand XSLT, but I've been building something which I believe solves a similar problem (albeit JSON-LD and Javascript). The general XML ecosystem of solutions have always looked really complex to me. You need to understand a lot more types/elements than I think is reasonable for people to author with but they are from before my time. I took a look at XForms 2 and it had its own way of defining functions which on top of the other XML quirks has security concerns.