Yes, volcanoes are relevant data points, but that's also very different to proposed methods of seeding. It does raise the question of whether someone will decide to set off bombs in fault-lines to try and set off an eruption - I recall Trump asking why hurricanes couldn't be disrupted by the early use of bombs.

> acid rain is a much smaller problem than global warming

Possibly, but acid rain can devastate marine ecosystems and cause issues with soil quality, so I'm not convinced that we can just hand-wave it away.

I can foresee greedy capitalists rubbing their hands together over being paid to seed SO2 into the stratosphere and then being paid to clean it up too. There's probably a lot of money to be made selling clean air if we can pollute the atmosphere enough to cause health problems for people.

The SO₂ cleans up itself in ~2 years.