It's not "Mammoth groves" if you want to meaningfully "buy time". A grove is an open tree stand a few acres wide. That's not what we're talking about. It's not adding a little bit of tree seed to unused spaces. It's inverting our footprint entirely.
It's planting the entirety of the UK, every year, indefinitely. It's employing a few million people with heavy machinery to walk from one side of the planet to the other and bulldoze every farmhouse they find, leaving behind mixed saplings at a wide spacing in the fields.
It's just way easier to decarbonize most of our society than it is to spend the land on forest carbon fixation. It's a lot of land, more land than we'd ever agree to dedicate.
And I would point out that planting trees and then sequestering that carbon periodically with charcoal production is by far the easiest way. Every other technique is ten or a hundred or a thousand times lower payoff.
If you want to slow things down, you have to mostly stop burning fossil fuels. We need to tax the hell out of them, we need to shut down new projects, we need to electrify anything feasible, we need to heavily subsidize and aggressively plan for rapidly scaling renewables. When we're at a 90% reduction in fossil fuel usage, then we can re-assess priorities because we've both "bought some time" and eliminated all the low-hanging fruit that's easier than deliberate carbon fixation projects.
More likely, we'll keep on steamrolling ahead, and something like sulfur geoengineering will be put forward as a way to stave off the worst effects for a while.