Plants release most of CO2 when they die. Locking CO2 in plants only works with actual living green mass. And we need a colossal and ever increasing amount of plants to offset our emissions. And plants needed just don't live in the tundra now.

As for easier food - tundra is poor environment, so it will be hard to grow anything there, even simply due to solar day/night issues. On the other hand, climate change on a scale when tundra becomes a place to grow food, would mean that some of the existing places to grow food could possibly stop being it. Imaging rice dies out in the hottest regions of the planet, where billions live. World war would be a child play compared to that.

Growing food is already dirt cheap, that's why we don't care so much about wasting 30%-50% globally.

> would plants and trees not take advantage and grow faster/bigger?

Not significantly, and it still doesn't solve any of the other issues, CO2 is but a tiny part of the problem, we talk about it because it's the only one we can pretend to be able to fix