Johnny
You have had some great advice already. A couple of new things that might not be readily identified in your research. There is a new customs dock in Victoria Harbor. It is in the same place but is much longer which should make checking-in much easier. As said below, you must go to customs first. Also keep in mind that Canadian authorities have very little tolerance for fire arms. They will let you register a long gun but will most likely make you give up any hand guns. Last year, we saw some folks face down on the dock and a large and expensive cruiser being towed away. The reason was that the owners of the boat did not declare their firearms that were later found on inspection. Outside of this, you will find the Canadian authorities very kind and helpful.
Victoria is a great city. There is so much to do in town. If you want to try an English Tea, you might consider the James Bay Tea Room instead of the Empress. The James Bay Tea Room is where the locals go (We used to live in Victoria). You do need to check with the Harbor Authority for space in the marina. There is so much going on in the inner harbor that the marina can be full at what might otherwise be odd times.
There are a few places not to miss in the Gulf Islands, they include Port of Sidney, Ganges Harbor (Salt Spring Island), Montague Harbor (You will have to anchor but the trip to the pub is worth it), Chemanus (The murals in town), Thetis Island and Pirates Cove. There are many others but this is a great start.
Although Victoria is a great city and I love going there, if I were planning a trip in to the Gulf Islands, I might consider giving it a pass. It really takes at least two days out of the trip, one to get down there and a second to get back to the actual Gulf Islands. If this were me, I would spend my first night in Roche Harbor and then go to either Sidney or Bedwell (might not be open in May) to enter Canada. If I really wanted to spend time in Victoria, I would rent a car and drive down for the day, leaving the boat in Sidney. I would then go in Brentwood Bay or Ganges next. If Brentwood Bay, my next stop might be Maple Bay or possibly Cowichan Bay. For the next stop, I would head for Chemanius (for the murals) and then Thetis Island. If you have lots of time go up through Dodds Narrows to Nanaimo. My next stop would be Pirates Cove or Montague Harbor. Finally, I would stop at Ganges. I would try to time my stop at Ganges to coinside with the farmers market. After Ganges, I would head back to Roche or Friday Harbor to clear back into the US. All of these stops are excellent for a several day visit. This trip could be done in a week but 10 to 14 days would be better. I took into account your comment that you prefer docks, so limited places where you would need to anchor. That said some of the best stops in the Gulf Islands are anchorages.
Good luck and have fun!
Shawn