SAXパーサーで全部メモリに読み込まないやり方すればどう？
XMLParserのinit(stream:)とかlibxml2とかで

https://developer.apple.com/documentation/foundation/xmlparser/1415904-init
https://www.raywenderlich.com/553/xml-tutorial-for-ios-how-to-choose-the-best-xml-parser-for-your-iphone-project