The problem with geo-engineering is that it only treats the symptoms, not the root cause. Treating the symptoms has a side-effect: it lowers the incentive to treat the cause by making it less pressing.
What we want to avoid is not a 1.5 degrees warming, it's a 3 or 4 degrees one. By doing geo-engineering, you could fix the issue in the short time but you would also increase the probability of a bigger problem in the longer term.