Okay, so what did Rhodes actually do?
Hard to say. Doing a Google search just now, I got four pages of MSM, Daily Beast, Vox, Salon, et al. The closest right leaning link I found was a NOV22 article from Cato Institute stating what his conviction was (sedition).
Wiki has this to say:
Stewart Rhodes. Elmer Stewart Rhodes III (born 1966) is a disbarred American lawyer, convicted felon, domestic terrorist, [3] and the founder of Oath Keepers, [4] an American far-right and alt-right anti-government militia.
The best I can see, trying to sift through the biased sources, is that he organized one of the protest groups, bought a bunch of guns beforehand, which was widely mentioned in articles, but brought none of the to the protest (not mentioned in articles). He was on Capitol grounds, but never in the building. He "obstructed official proceedings". After the protest, he was apparently recorded saying that he regretted not having people show up armed, and something about hanging Nancy Pelosi from a lamp post.
The most I could see out of this would be 18 months, not 18 years. Apparently because a judge declared him a terrorist, harsher punishments were made available for what otherwise would be misdemeanor trespassing (there are hundreds of incidents yearly involving protestors "obstructing official proceedings" in state capitols all over the country). Nobody got 18 years for it. He didn't even destroy any property from what I read, so they couldn't even get him for destruction of government property.
If talking about hanging Nancy Pelosi is a crime, there are hundreds of thousands of people that need to be arrested for saying the same about pretty much every notable politician ever.