I recently came off a project with Ember, React, and LiveView all working simultaneously. Part of my job was to help the effort to finish converting the Ember stuff into LiveView so Ember could be dropped entirely. We did it like you said, in small chunks.
It is doable. I did not configure the parallel setup, so hard to give any firm recommendations. I recall we configured the React router to be the first opportunity to match a URL request/path, and then if none could be found we would then let the Phoenix router attempt to handle it.
One other idea/note is: you might want to (at least at first) build your LiveView to use the same GraphQL endpoint as the React experience used to make sure to maintain the domain/data layer as close as possible. In time you might replace that internal use with more traditional Phoenix business domain contexts.
Good luck.
