Not that they'll necessarily succeed, but just try something.
Think about it: These people literally, unironically believe that Biden/Harris are Satanic, cannibalistic, commie pedophiles who illegally stole the election from their God Emperor (Appointed by Jesus himself) and are going to round up and execute all Patriotic White Christian Americans. These people also have way more guns on average than normal people due to this psychotic paranoia.
Are all of them really just gonna stand by and "let that happen"?
Personal prediction: At least one chud will be detained by the Secret Service for trying to get in to the inauguration with a pistol or something, but no one will be seriously hurt or killed, much less Biden/Harris.
I was thinking a 500g bloc of explosive surrounded by nails/bolts and the like would be more than enough (and small-ish amateur drones can easily support such a payload weight - battery life doesn't matter anyway, even if it's 10 minutes that enough range to make sure you're launching from a safe point). You're detonating this in a sphere of around two meters at worst around your target in my scenario; even a shitty explosive like TATP seems like it would work. Maybe I'm wrong and I highly overestimate how those explosives perform ?
As for commercial availability, it doesn't matter, they're easy enough to build by yourself. And a tilt rotor drone (or even just an old-style radio-less independant plane with ardupilot) would go fast enough that reaction time would likely not be adequate - neither from the target itself nor from his/her security detail.
You probably are way overestimating how well explosives perform. They need something strong to hold them in and let pressure build to be effective. Grenades are usually made of relatively thick cast iron or steel. This is heavy.
As for security reaction to such a thing, there is no doubt they have gone through the possibilities and have some sort of antidrone defense in place. They already plan for snipers by obsessively sweeping a wide area, so the nearest safe area to launch a drone would be far enough away they would have plenty of time to react with whatever specific weapon systems or countermeasures they decide to use, and get the target into an armored vehicle.
That's interesting; I'm still not fully convinced the concept is entirely not viable but thank you.