1-I would look hard at the tires as part of the suspension. Tires that run around 25-30Psi MTB/Cruiser sizes) and Tannus liners (not only prevent flats but absorb bumps and provide some damping that just the tires don't). Do as much as you can to get things smoothed out before that big hub starts moving around. I've had very good results on hardtails with this approach. This is how Older American luxury cars got away with heavy rear live axles, and squishy springs for so many years
2- Engineers look at this as a weight, and a spring that produce a certain frequency. There are formulas and calculators for this.
Sprung weight (weight on that axle) unsprung weight (The hub and suspension weight) Desired frequency (comfy,sporty,HD?) and spring rate ( adjusted for suspension ratio) are the variables.
This may give you some ideas of which way to go from where you are. Shock absorbtion comes down to limiting the number of cycles that occur per bump. Probably much stiffer to control this hub. The ratio of motion between the spring and axle is important because it makes the spring rate exponential to the leverage. 2x lever=4x spring,3x=9x. etc. Forks are 1:1
3- Taking this further, it's normal practice to have a faster frequency for the rear than the front suspension. This allows the rear to "catch up" to the front over each bump based on the wheelbase and speed. This has a lot to do with fore and aft pitching movements, which on a bicycle are actually very large. You may actually achieve this with just the tires at some speed over small bumps at that speed. It's nice to be aware of this in case it happens for you. Tire pressure can tune the speed up and down some.
I don't think anyone can answer this for you. But understanding the principles can get you headed in the right direction.
I'm going to say a little more about #3 above. If you look at the problem as a force applied at each tire contact point, acting upon the riders Center of Gravity you will have a triangle rocking front to rear several times over each bump. If you can tune this pitching moment out, then you can work on a simple vertical force that most people think is the primary problem. A comfortable frequency would be the goal there. If you can do this at the tires then you can gain some flexibility with the spring rates. You can lose this with too soft a rear spring. Maybe lock out the rear suspension, and tune the tires as much as you can. The trend will be towards a stiffer rear spring, both to control the weight of the hub, but also to catch up with the front end. Of course the front can be tuned some also. The heavy rear spring will also allow some reserve capacity for a passenger, or luggage if needed.
I don’t know about changing the whole suspension, but definitely re-valving. I would think, especially the high speed compression needs to be stiffened to help with the upspring of the heavy hub motor. Your rear shock is valved for the factory weight and with the heavy hub motor I’m sure it’s running through the suspension on hard hits.
I was actually thinking about this earlier with heavy hub motors on street bikes, that it could probably be dangerous in the turns if the rear suspension is not set up properly. If you hit a large bump and the wheel is bouncing up off the ground going through a turn could be dangerous.
You need slower rebound to avoid the pogo sticking effect of the motor reacting against the swingarm. It’s a compromise between that and rough road handling. The high unsprung weight is another problem that you’ll experience as a wheel stutter at high lean angles. You can try messing with the tire pressure but my experience has shown me to keep it in a suitable range for the tire and to instead mitigate rear hub issues with shock setup.