My reponse is kinda long so I have broken it up into two parts One I think this is just a good example of how just because we can do something doesn't mean we should, no matter what the good intentions are, when we have no idea about the effects that will have.

The cloud coverage is an issue that has pretty much become foundational at this point just look at the winter snow melts on sunny days without cloud coverage even if it is only a little bit. Sulfur dioxide is a known cloud seeder and emission reduction of sulfur was highlighted as possibly a very bad idea years ago. MIT even had a think about it which you can find here.

There are tons of other issues we could target first from plastics to coal usage etc. We plain chose an awful one to target first.

Two Its not exactly true/accurate we are the only being that harms and damages the environment. Even if we cause a mass extinction we wouldn't be the first according to the fossil record. While modern-day Cyanobacteria are fine 2.45 billion years ago when they caused the Great Oxygenation Event they "destroyed" the planet at that time because what was around could not handle high oxygen levels.

Another example is Ants while they can do a ton of good when you have ones like the Red Imported Fire Ant which has spread outside its natural habitat of South America. Now yes human sped up their spread but due to how they form balls and can float in rivers, lakes, the oceans and survive up to 12 days. They more likely than not would have moved into North America and the Caribbean over time without the need for humans due to weather patterns. Now getting out of the Americas and to Asia, Europe, Africa, Australia, etc would be hard its not the craziest thing to think with some freak weather or environmental event that they would have gotten there and wreaked havoc