Skip anything else when time is limited and hit the makiwara. Water seal the base of the makiwara before you drop it in the ground. Don't ever use pine to make a makiwara. Cedar is acceptable. Don't make the makiwara too stiff.
Flexibility is good.
The goal is not for it to hurt. The goal is for the device to give, and then for the resistance to increase as you press in on it. This is different from a bag in that the bag offers initial resistance, and then caves in under pressure. Don't make it too flexible. Stiffness helps hip rotation.