Another point to consider is that in order to begin or complete a spin the plane must overcome it's rotational inertia.

Center of Gravity (CG) is all the airplane's mass theoretically compressed to fit inside a point and located at that location to substitute for the mass of the entire airplane and this is great for describing behavior when flying without rotation.

However, when the plane begins a spin, mass and CG aren't enough info. You also need to consider rotational inertia (I) which is a theoretical ring of mass (like a giant wedding band) equal to the entire airplane mass with a radius of gyration (k) about the rotational axis which could be substituted for the airplane. This inertia is different for each axis. The equation is: (I = k squared times m) where m represents mass of the plane so (k) is most important. To be clear, (k) would be the radius of that wedding band.

A typical spin is a combination of a barrel roll and a flat spin like sliding on the airplanes belly sideways inside a giant funnel. Airplanes are well designed from a rotational inertia standpoint to begin and end an axial roll. Most of the mass (engine, batteries, fuel) is located right on the axis of rotation (k=0) and wing panels don't weigh very much. Combine this with the highly effective ailerons way out on the tips of those wing panels and you have a very effective system. Some Ultimates can roll so fast they blur! The only difference between an axial and barrel roll is Normal Acceleration which I described in a recent thread so I won't go into it here.

The flat spin is a different case altogether. Much of the weight (engine/fuel) is a significant distance from the rotation axis resulting in a larger radius of gyration (k = 4"or so) which in turn, requires more force to begin and end the spin. Now consider the control surfaces. Only rudder is well equipped to apply force in the direction of rotation. (For those eager to correct me, yes I know its a moment and not a force but it just complicates things.)

How can you improve flat spin performance? Reduce rotational inertia and improve rudder effectiveness is a good start. Try locating the fuel tank directly over the CG. In addition to reducing (k) it will eliminate trim variation with fuel consumption. A shorter fuselage will locate the engine mass closer to the CG while a larger rudder with more deflection angle will move more air and compensate for the shorter tail moment resulting from the shorter fuselage. Also, any general weight reduction will improve all performance characteristics including flat spins but you'll want to start with the heaviest mass furthest from the rotational axis.

Realize that no matter how much you reduce rotational inertia there will always be lag because some mass is present. You have to anticipate and learn to compensate for this. Practice making the spin with the same speed and attitude for consistency. If it takes a half rotation to recover after pilot input then start the input that much earlier. Because rudder surface area is never symmetrical to the longitudinal plane of the CG you will have to compensate for roll while performing the maneuver. Adding a sub-fin and lowering the rudder will help reduce this.