There are two different bus lines operating between Nanaimo (Departure Bay) (ferry to/from Vancouver) and Buckley Bay (ferry to/fom Denman/Hornby).†
Not a lot of Greyhound options left in BC these days. These guys took over the defunct Island Bus Lines routes. If you want to check the schedule or book online use the Greyhound website.†
The bus runs to/from the Nanaimo bus terminal.† There used to be a bus to/from Departure Bay (the ferry) from/to Courtenay, but it's been discontinued.† There is a public bus from Departure Bay to/from the Nanaimo bus terminal, but it's ill-timed and distressingly slow.† You could probably walk the distance in about 25 minutes. Note that the bus from Vancouver goes to the depot if you took it on to the ferry.
Book to/from Buckley Bay.
My recommended way to go. This is a local firm that schedules its buses so they make convenient ferry connections. And if the ferry is late arriving, the bus (actually, a passenger van) doesn't leave until after its in. Wifi on all buses Ė and they will transport a bicycle if itís boxed.
Book online - the website for bookings or just schedules is http://islandlinkbus.com
To or from the Horseshoe Bay terminal, take the #257 express bus.† Downtown, it stops in front of The Bay and a few more places.†
I personally leave home on foot, hop the Canada Line to downtown, transfer to the #257, walk onto the ferry at Horseshoe Bay, walk off at Departure Bay (Nanaimo) and get into the waiting Island Link bus, get dropped off at Buckley Bay, and canvass the drivers in Lane 1 of the ferry waiting area for a lift to Hornby (at least, thatís what I did before Covid-19).†† Door to door about 4.5 hours, without a car, if you do your planning.