Lose the birdshot sizes entirely...they don't penetrate well enough for self defense. Many of the pellets may not penetrate deeper that 4-5" (shown in gellatin tests) and that just isn't good enough to reach the vitals. Sure, it will make a very large wound but it will be shallow. I would start with and stay with #3 buck as this is regarded by many experts as the best defensive load for the 20 gauge.
If you live in a place that causes you to worry about overpenetration, then you can use the lighter loads, but be prepared with extra buckshot as your second or third rounds to come out, as the birdshot may not stop a crazed or determined attacker. The buck will. Only use slugs if you plan on doing any shooting at a distance. You might keep a few in a sidesaddle or in a nylon carrier on the stock. But buckshot is usually far more useful and practical in a home defense package.