Why should we sharpen our gifts, skills, abilities and character now while on Earth, partially in preparation for Heaven (where those gifts will be used for Christ), when we'll have eternity to do so?
Part of the issue is why should we be working on something now? Because this life is a limited opportunity to make choices to follow Christ in a world that is opposed to Him. The idea of why not wait until eternity, since we’ll have all eternity to learn things, is answered because God says not to wait. This is our opportunity and God will reward us based on our faithfulness to the opportunities given us.
You are neutralizing the doctrine of rewards without realizing it based on your concept of infinite joy in Heaven.
Faithfulness to Christ in this life allows us a head start when we enter eternity and we’ll be further along when we start. Even if our knowledge and skills will be multiplying from where we left off on Earth, we will start further ahead. For example: ruling over five cities: that’s where we start (five cities is the first reward) and then grow from there.
For more information on the subject of Heaven, see Randy Alcorn's book Heaven.