Yep at this point we just have to do whatever seems least risky. It's not like we have a pristine planet. We have to balance the unknown consequences of phytoplankton (and other mitigations) against the known and unknown consequences of a >50% higher CO2 level and >1.4C higher global temperature.

There are other solutions that require mass coordination and possible violence that would be a lot easier than gambling on phytoplankton