Interesting, complex solution to using an SMB...only problem is, it assumes that one is only going to deploy one from 5 meters. Reels are used because they are simple, allow an SMB to be sent up from any depth one has line for and don't take much space.
If you are using a large smb (ocean, big waves, currents, etc), then it is neither practical nor advised to send one up when shallow. Deeper allows more air expansion, which means you can fill it quicker, with less gas pulling you up during the fill process (talking over 2 meter smbs).
What if you are doing a mid water safety stop?
What if conditions at 15 ft are not suited to deploying an smb? (we tend to have major surface currents, but calm at depth)?
From what I can tell, your solution would work for deploying a small smb, in calm conditions, on a simple dive with no extra stops. I would guess that would represent about 20% of the dives I make.