Yeah, very stable. Its actually slightly diheadral, not flat, which also helps stability. A big delta wing is the way to go on an efoil that doesnt need to be very nimble.
Foils dont really care how much you weigh. Well not as much as you would think. It just means you would trim the stabilizer a bit more than me and run at a lower AOA. Im 95kg and Im assuming you are less.
Im not sure how fast Im going when it foils up. maybe 15 kph. Foiling below this speed would be pretty boring for me. I tend to just ride the board on the water and jump on the foil when im ready.
I think there is merit in riding a wing like the Naish thrust surf foil, if cruising around is your style of riding. My build was always about finding a top speed.