Small payloads are not a problem if you’re releasing the right stuff. Antrhax spores aren’t particularly heavy, and several kilos of them are plenty if released in the right spot. They’ll be detected at some point, and anthrax isn’t particularly communicable. There are some worse things that could be used instead.

A low-observable carbon-fiber UAV launched from a ship offshore with an internal combustion motor and a secondary approach electric motor could fly at 10,000′ to target, glide down to 500 feet or so, and then quietly dust Central Park or the National Mall with a biological payload one night, and I doubt anyone would be the wiser. Crash the UAV into the middle of the largest adjacent body of water and it may never be found. If the jihadis were smart, they would hit Boston instead of New York, and Baltimore instead of DC. Or if they had the range, Richmond and Harrisburg. I think most people know cities are targets now. What they’re not expecting is for the enemy to jump 50 or 100 spots down the target list and do something nasty.

Drones aren’t a strategy, they’re a tactic, and I agree that eventually everyone will have them because the technology is so available and reliable, unlike specific isotopes of uranium and the products of fission. You don’t need Oak Ridge or its equivalent at all. The biologics are pretty easy to come by and we know from Army experiments in the 1990s that any moderate-sized metal building can be a production site with COTS equipment. There has already been one chemical weapon terror event (Tokyo) that only managed to kill and maim a few dozen because of a poor delivery system, and one biological attack that was limited only by the scope of the attacker’s anger. I don’t think it’s a matter of if, but when.