It's well known that large volcanic eruptions have caused global cooling for a few years that's severe enough to lower the temperature of earths surface several degrees.

I don't think anyone can deny that it's possible to cool the Earth by putting stuff in the upper atmosphere because we've already seen it happen during recorded history.

The Tambora phenomenon is well documented. But that does nothing to solve other problems like e.g. ocean acidification, and potentially causes many problems of its own. It's a really complex system and we need to spend way more time and money understanding it than we currently are.