I don’t know any launch sites on the north west side of Galiano. Portier Pass Rd always looks to be a possibility but I’ve never tried and I’ve not heard of it being used.
My preference would be setting off from Salt Spring Island, driving north from the dock at Long Harbour and launching from Hudson Point.
https://www.bcmarinetrails.org/map#marine-trail-sites-within-bc-parks You’ll need to accept the terms to view the map
There is a ferry dock at Vesuvius, on the west side of Salt Spring, and a beach that’s accessible as well but you’d need to check on overnight parking before trying there. It might be your best bet or else you’re only left with Montague Harbour.