Do you feel that you need a PFD on a lake? I'll confess I didn't use one last summer on a trip in the Wind River Range. I pieced together a route that was arguably safer on the water than the adjoining scree field and cliffs. I tried to stay a few "dog paddles" from shore, and stayed out of the wind. I felt safe...although I admit appreciating the durability of the Scout, I paid so much for, when I knew I was in deep water!
Looking at your lightweight setup, I'm wondering if there isn't a dual use PFD/sit pad/pillow on the market like what you've done. MLD no longer sells their The Thing. I have seen some of the inflatable PFDs, but no one seems to provide the weights and they all seem to be overly complicated with pockets I don't need.
Interested in what you have tried for lakes? I travel solo, so I don't try fast water.