I own the large SM F-4 and the moment it came out of the box I could see the proportion of the nose in relation to the intakes is wrong. Then I had to pull out the Tamiya 1/32 scale model and one of my Phantom books and there is a clear problem with the barge boards. Im not sure what the scale source of the master model was but its a shame this nice size F-4 is not as accurate as it could have been.
Should have used a Hasegawa 1/32 Phantom instead of Tamiya! Lol
Anyway, I will build this jet eventually even though it erks me to see what cannot be unseen. Ive considered what it would take to fix the front end and there is no point it is what it is, just fly it.

As for the slats in a fixed position I would agree that either fixed deployed or fixed retracted as long as there is an open slot between the slat and the wing it will work. Make sure the slot is clean and not blocking the flow of air. The inside surface of the slat element should be smooth and mirror the surface of the main wing.
I have been testing this very same concept on a very small A-4 EDF for over a year so I could understand better high AOA jet flight. Its amazing and really opens up the flight envelope of the model. I use flaps as well and I can do figure 8 touch and goes while keeping it at less than 6 feet altitude. The top speed does suffer but the slow speed capability is the key and this is where you have to get used to flying slower than your used to doing. The SM F-4 does have very stable and impressive slow flight characteristics but I could only imagine based on what Ive done so far that it would just allow you to land at a walking speed practically if you added slats. Great for short field use! You will also have to understand your turbines spool up speeds and how the jet responds in turn because high AOA flight with a jet requires your immediate input.
I would prefer to make the LE Flaps work rather than the Slats static but thats just me.

Good luck, love to see your progress. Maybe this will inspire Anton to do some updates because the F-4 is a very popular and historic aircraft.