Your idea doesn't make any sense. So you have the option of:
1. Short charge in the first stage, but charging during the second stage is slow and weaker
2. Long charge in the first stage, but second stage is faster and stronger.
That doesn't make any sense. You are spending equal amounts of time in both situations. If both beams take the same amount of time to charge, why even short charge at all? You might as well long charge and have some kick in your beam.
The way it is now is just fine; the longer you charge the more powerful and bigger radius your beam will have. You can even hit turbo for added power. Maybe if the first stage of charging took a long time, but added power and took no ki, then there would be a good reason to use it. FF could be the PS dominator then...
Keep thinking tho, beams could use some depth.