If you're limited to 2.5 inch shells, Winchester has an excellent load (XB41000)which uses three 000 Buckshot. My wife handles it easily and it seems very adequate for "homeland security" in our Winchester 9410. They are available from most of the internet sales operations. I've bought them from Natchez.
Also keep in mind that at short ranges, even birdshot, by not having had much room to expand it's shot swarm is very dense, and will pack a much bigger punch than most people realize. Also, while devastating at close range, birdshot loads will minimize the hazard of overpenetration.
About 30 years or more ago when I was living in FL an aggressively hostile drunk tried to break down the front door of an older couple's house in the wee hours of the morning for some unknown reason. After warning the guy to back off several times (according to the old guy's testimony that is) the homeowner fired a round of .410 birdshot through the wooden door and took off most of the lower part of the drunk's face. He survived after a long stay in the hospital at the taxpayer's expense, but his face was a real mess as long as I knew anything about him.
Don't take a .410 lightly, at close range it can be deadly even with small shot, and as you said, triple ought buckshot is also available in the .410, and 3- .36" diameter lead balls moseying along at 1200 fps is nothing to sneeze at. Wild Bill Hickock put down quite a few men with a .36 caliber Navy Colt that fired just one .36" round lead ball at a lot lower velocity than the .410 buckshot load.