The FBI report didn't fully (or even really partially) investigate the person / people who instigated the action. I keep coming back to Trump because he's the person who fomented a coupe attempt. The only reason that these people aren't being charge like it was a failed coupe is because the Republicans in the Senate allowed Trump to walk. You say that the FBI didn't find evidence of origination / coordination to insight violence. They just found evidence of coordination to GET ALL OF TRUMP'S SUPPORTERS IN UNIFORM, IN ONE PLACE, SO HE COULD ADDRESS THEM. Of course, when he addressed them he told them what he wanted as did his cadre of middle men.
Don't try to tell me that this was an army of lone wolves that all decided to attack the capitol individually. That's not how these things work.
Even if they are all considered to be a conglomeration of lone actors... that still doesn't excuse the fact that they each individually chose to assault the sanctity of the votes of millions of people through violence and destruction. You aren't allowed to intimidate someone physically at a voting booth. Why are you able to do it to their representatives and get a lighter sentence?
80 Million people had their votes threatened that day. 80 Million. FBI report be damned. That's worth a lot more than a few years in prison, unless we as a society should decide to show mercy.
The same would go if those protesters or their representatives, had been able to prove with any tangible evidence that their votes had been nullified by election scams. To date they never could.