I would strongly recommend against doing anything new in XUL.
As you say, React is already in use in Zotero — the tag selector in
production is built using React, and a React collection tree is almost
ready. These use standard React/HTML. As long as you limit your
dependencies, Electron isn't a dependency here.
The main catch is that we don't yet have React versions of some of the
core UI building blocks, but you can rely on built-in functions for some
things, like progress windows:
(That currently requires bundled localized strings, but we can change that.)