I do wonder if some of the "massed solar sail" ideas for terraforming Mars' atmosphere would work here (in short: use cellphone processors to build and launch a couple hundred thousand solar-sail equipped satellites).
Because it would be expensive, but it would kill several birds with one stone: we (1) prove whether the concept would work on Mars, (2) develop the technology to do it, and (3) unlike SO2 in the atmosphere, "switching it off" or modifying the scale of the effect can be done almost instantly (you could remove the swarm by having it fall back to an Earth orbit).
Is there anything left to really "figure out" about carbon capture? The tech works, it's just too expensive. Given a sufficient amount of cheap enough energy it could be scaled up as far as I understand.
As a layperson, it's just one more reason I so wish we'd invest in nuclear. Nuclear powered DAC plants might be the only way to scale it fast enough. Sure, it would still be expensive, but that's much cheaper than not massively reducing CO2 in the atmosphere.
Based on our record of environmental interventions; SO2 sounds suicidal. But it only lasts a few days in the atmosphere so maybe it has an in built off switch.
Note that is for the stratosphere, not the atmosphere.
The mechanism is well known from volcanic eruptions, and it only lasts (from memory) 1-2 years in the stratosphere, so if something goes wrong, it can easily be tapered off.
Because it would be expensive, but it would kill several birds with one stone: we (1) prove whether the concept would work on Mars, (2) develop the technology to do it, and (3) unlike SO2 in the atmosphere, "switching it off" or modifying the scale of the effect can be done almost instantly (you could remove the swarm by having it fall back to an Earth orbit).