TLDR - Could Apple do this with a quick update - of course it’s technically possible. Will Apple do so? I prefer the odds a snowball survives in December in Australia, but who knows what new hardware will allow or if software in the future is allowed to spend watch battery on this feature.
On a technical level, with the current iOS/iPadOS/watchOS throttling background data connections at 30 seconds in general when it used to be 10 minutes, there is no way you’ll be able to fit that much data through the watch hardware. My opinion is the carriers are borderline cruel to even offer that much “watch data” in my opinion let alone charge you $120 per year plus fees for that premium data. It would be a triumph to exhaust that data cap with the hardware and software going into 2020.
You are correct that if the watch is on WiFi - it will tunnel all data over that and if that doesn’t work, then the watch will talk to your iPhone over bluetooth to avoid running the LTE on the watch.
WatchOS will only consume cellular as the last resort and certainly the tethering code is disabled on watchOS as well to save power and allow it to run as described above.
If cost savings is your desire, consider dropping LTE on the watch and seeing if you an manage without it and fund more phone data.