I suppose it depends what audience will be editing it. XUL used it because that was the fashion at the time, but it wouldn't be something I'd want to hand edit. Qt used its own QWT (now dated) which was a lot easier for a human to edit, albeit a bit ad-hoc.

If it's mostly consumed and edited by a machine/LLM, xml might actually be ok. Otherwise, I think something a bit less ceremonial. Obviously there's lots of options, and I imagine you may even miss much of the tooling of XML (schemas and such).

Kind of a drive-by comment, I admit, but it just struck me as a little surprising.

Also: it's been some 3 decades since I was playing with Ada ... very cool to see it getting some GUI love!