A day later, and I can at least.. read the actual XML data from KDBX vaults.
Then there's still actually parsing the XML, and writing it all back, and hooking it all up to some kind of UI... but I'm honestly also kind of surprised that I at least got this far without running out of steam?
I suppose good docs help, and https://keepass.info/help/kb/kdbx.html was surprisingly helpful to deal with. (Despite running into a bunch of oddities and windowsisms.. I honestly expected far worse, given its age.)