Geoengineering is really the only realistic option if this is the case.

Sulfate aerosol injection is by far the most promising. And cheap - maybe 5-8b per year [1]. I'm not claiming this is a good idea in any way. As they say: desperate times call for desperate measures. We could buy ourselves several decades to sort our (as humans) shit out. We don't understand the unintended consequences very well, but at least this technique mimics natural processes (volcanic eruptions).

[1]https://iopscience.iop.org/article/10.1088/1748-9326/7/3/034...