The single best thing to do would be to bury the tops of the tires into the wheelwell arch.
It would help the looks and the aerodynamics, so win-win.
Beyond that , It's actually a pretty clean shape. The 'low-hanging fruit' would be the underbody and rear wake. The inner half of your front wheel spats aren't doing what you hope, but they might not be hurting. The side skirts aren't a bad idea, with or without lowering.
Airplane hangar door seal comes in 6' lengths and 2 to 6" depth. Bajoos' Aerocivic has a dual side skirt flush with the inner and outer face of the tires. So you could do a 2" outer skirt all the way up to a dual 6" skirt. Flexible nylon brushes would take a lot of ground strikes easily. If it is used across the vehicle, like an air dam they deflect; but longitudinal 2-3" brushes shouldn't wave around like a grass skirt.
Rear (and front) fender skirts and a rear diffuser, and a full bellypan with horizontal brushes to inner faces of the front and rear tires. For the rear, a nicely integrated box cavity:
