I am not a bunk trailer guy, but I always hear that dunking the bunks to get them wet helps the boat slide a bit. You need to find that sweet spot for your boat and your trailer of how far to back it in. As you stated, it shouldn't be so far in that you're knocking into the spare tire.
Every lake is different. I put grip tape all around my trailer frame so that I can safely walk on it in wet conditions. A good set of water shoes may be your friend as well if it is still difficult to load. To me it's like portaging with a canoe. If you expect to stay dry, you will eventually get soaked. If you just resign yourself to getting your feet wet everything will actually go much better.
All of that said, backing in to the proper depth and a good dock should mean that you never get wet. I have a dedicated docking line that I use to load and unload my boat from the trailer. It helps tremendously and saves me from needing to powerload - which is harmful to most boat launches.
