- Edited
@MetropleX Okay, tried again from a fresh install and followed everything step-by-step as described above and still continued to encounter the same issue with English (US) and (UK).
In the end what fixed it was disabling "Multilingual typing" found in Gboard settings > Languages. I think this feature allows you to use autocorrect while typing a different language than the one of the keyboard, perhaps it creates some sort of dependency between the different languages installed and prevents offline voice typing from working correctly if the second language isn't also available offline (in this case French). Option might not appear if you only have one language installed.