The important, missing detail that breaks down this analogy is that we don't have a reference for a long period of vulcanism while anthropogenic emissions of greenhouse gases continue.

This is where the "termination shock" issue comes in. Given current CO2 emission rates, a 50 year geoengineering strategy would mask an additional 100-125 ppm of CO2 added to the atmosphere. If the geoengineering scheme was suddenly stopped, it's not entirely obvious what the response trajectory would be of the climate system.

Yeah, stratospheric seeding is mostly a stopgap measure that needs to be used in conjunction with other geoengineering and political projects that strive to reduce/reverse CO2 emissions.

And therein arises the "moral hazard" issue. There is a legitimate concern that geoengineering could abate some of the concern over climate change and lead to further delays to reduce GHG emissions. And this is a serious problem because while we might mask global temperature change with these Approaches, they don't help resolve issues like ocean acidification.