Hi,
For now I'd definitely move to the flooded setting.
Since you already have the remote for the Magnum I'd lean towards the PT-100. Best price for that may be at Imarine.
I agree that an 80 amp controller will leave no room for growth.
I'd likely put all six panels in series. I know that is a risk for shading. It could easily be undone since you have adequate wire already in place. If you added 2 more panels, I'd do two strings of four in series/parallel.
The other option is to switch the battery bank to 24 volts (or even 48), and add a Dc to Dc converter for the (minor) 12 volt needs. Unfortunately that means replacing the (lovely) Magnum 3012. That is a bit of an "ouch", unless you can find a buyer.
