Well after a night of tryout :
Read from the union mount is solid, up to 5 concurrent streams without any issue whatsoever, the overnight plex library scan went fine but took slighly more time than when it was reading from Mergerfs. (we're tallking about 30 more seconds which I think is completly ok)
Deep scan of a manually added mkv went faster than Mergerfs though.
I gave a look at the google dev console to check the api key metrics, looks exactly the same than when using mergerfs so I'd say we're safe from rate limits bans (I made new key to be able to tell the difference).
Write to the union mount on the other hand is sketchy at least with my current mount settings (same as yours). Couldn't figure a way to let radarr import a movie to the mount. (I need further investigating this, may be a persmission issue) And when trying to mess with mount settings (playing with dir-cache-time mostly) I even managed to prevent myself from writing to the mount via SMB at some point (from my hackintosh AND from a regular Win laptop)
Did you have any more luck?
Disclaimer : I not a dev, I'm just geek enough to understand your scripts and tweak them to my liking but couldn't write them from scratch. Just so you know I might not be of a big help here...
EDIT :
Further testing and wondering...
Sonarr seams happier with the union backend, it imports files fine to the union mounted remote but files are being written straight to google drive 🤔