Those are pretty good readings, makes sense that the DO is low below 8 feet, what time of day did you take the readings? Try taking some at night as well.
I wouldn't put the diffuser on the bottom - with DO that low you could force a fish kill, check youtube for that exact same scenario..
I think you might have similar results with no aeration but that is just me, my lake is the same size and depth and I have no aeration, similar DO results. Honestly dont over think it, your fish are super healthy from what you have said so other than water clarity and algae which you have started to clear up why bother throwing a wrench into the mix.
Check out my recent post on riparian/hyporheic zones, that might make a difference for you was well.
