@freemo
What you describe seems to fit with a very general "left I don't like" approach, as it definitely does not describe a single coherent political movement. I wasn't aware you were also using "alt-right" in the same sense though (or that it apparently had that meaning originally?). It's still confusing, because "alt-right" has a meaning referring to a specific movement too, while "alt-left" doesn't.
It also explains why many people who are called "alt-right" and deny that label, while all people called "alt-left" deny it.
@freemo
It is (or at least was in the early 2010's when I interacted with them) not a small movement. And it is definitely more than just not agreeing with the right mainstream. They support the idea of an ethnostate, generally believe in dangers posed by global warming, strongly support free speech in a way that explicitly includes verbal harrasment. The latter is the source of so many people being annoyed by mentions of free speach recently (see also "freeze peach" in mastodon instance blocklists). They used mostly internet activism, originally mostly in the form of trolling, although later shifted to other forms, which broke the meaning of "trolling", because the media couldn't keep up. They seem to also believe in accelerationism. If you want a very extreme, but accurate description of their beliefs the Christchurch shooter's manifesto is a reasonable source. Obviously, extreme, so many self-descibed alt-righters won't agree with his methods, but the gist of the ideology is there.
I know about this because I read many discussions around the time the movement was being born, both from its proponents as well as enemies.