I think there is only one possible route, which is the alternate route shown by Google Maps, i.e. going back to Hwy 19.
Google Maps shows you that there is a possibility to take a car ferry to Port Eliza and then to Zeballos. It tells you it is a BC ferry however I could not find any trace of such ferry on BCFerries map (maybe it is only running in summer but I still could not find any). And by searching online, I could not find any other ferry company offering the ride. Anyway Google predicts both ferries take between 1h30 and 2h (and its 3h30 total estimation is therefore extremely optimistic as it probably takes a while to load and unload the car), while going through Hwy 19 is predicted as 4h30, which in the end might not be much longer.
OpenStreetMap shows more roads between Tahsis and Zeballos, that seem to be logging roads (we can see some on the satellite image), but no road that links both communities. Logging roads are probably not recommended. I remember driving on one on Vancouver Island and while this was possible, it was slow and I was mostly worried by fast trucks not having so much visibility due to the dust.
So in the end, unless you absolutely need to find the shortest path (and are ready to discover the area by trying or at least asking locals if such possibility exist), you should probably choose the safer option of going back to the Hwy 19. Note that this area is quite remote and therefore access is not easy and may require patience.