bioengineering more heat-tolerant plants and mechanisms to carbon sink seem to be more easily solved than bioengineering radiation-resistant humans, but then again predicting future development is almost impossible.
On the other hand chemistry is highly temperature sensitive and at some point proteins will just denature. Then again you cannot have rapid growth without water, as plants need evaporation to transport minerals and such from the soil into the plant. And what about phosphorus anyway? Soon most soils may be nothing but dirt. You cannot work around the phosphorus erosion. Once it's lost, it's lost. We better start recycling our feces and the dead now. You kinda need to harden all life, not just humans and crops.