leftist can be thought of as just another flavor of authoritarian

Almost all leftists will say the end goal is communism / anarcho-communism. A system in which the workers control the means of production and anyone involved in production gets an equal say in how that production is done. Before this can be achieved the industrial capacity of a country has to be developed enough to easily provide for everyone's needs. Marx thought that once everyone's needs were met consumption would plateau and therefore so would growth. Capitalists would then compete for a fixed size pie where the only way to make money would be squeezing workers and automating jobs away, which would reduce the amount of labor needed but cause mass unemployment. This would eventually reach a tipping point where the workers would rise up, seize the means of production and redistribute resources so everyone go there needs met while working far less, and thus relieving the intraclass tension that would tear it apart once it took over.

The problem is capitalism is very good at creating new needs for people and instilling the desire for them in the working class through media and advertising. So the growth keeps going and we never reach that end state.

These needs and desires are built deep within most people living under capitalism at this point so even if a socialist revolution did happen, which is unlikely due to the above reasons, then those desires would still show in the workers choices on production, so we'd still be making f-150s, hamburgers etc.

The only way I think could result in degrowth is a massive re-education program to remove those desires, most likely through an authoritarian media control. You'd need to counteract all the advertising people have ingested, think of how much of your life you have spent watching car commercials. A person would have to watch just as many hours watching videos on how to destructive and environmentally disgusting cars are to counteract all the positive associations ads have instilled in them. Along with replacing ads with environmental psas, media would also have to be mandated to reflect environmental values, driving and eating meat would be shamed and only done by the lazy and cruel.

I'd say this is definitely authoritarian, totalitarian even, but not fascist. I struggle to think of a coherent eco-fascist ideology as fascism is all about ethno-nationalism, national supremacy and expansionism which don't work with any environmental goals. so I think Eco-fascism is just a derogatory term used by people who don't understand what fascism is and just think all evil authoritarians are fascist.

No. I oppose authoritarianism in all its flavors.

I also don’t think our species will ever willingly do “degrowth”

I think the collapse of modern civilization is an inevitability, and that there will never be any positive, organized way to guide that collapse to any worthwhile positive outcome. Will it come in my lifetime or yours? Probably not. But it will come.

The only wildcard I see that could change this would be fusion energy, actual self sustaining fusion that generates unlimited energy, or the discovery of some other exotic form of matter that can provide unlimited energy. Barring those highly unlikely developments, I think any form of eco-authoritarianism would just be a way for those in power to continue enriching themselves at the cost of everyone else, a more extreme form of what is already happening.