[xophere]

Do you think it worth trying a progressive spring?

I think the problem lies in the direct cantilever construction, which will react less progressive than a more modern suspension with linkage elements that will emulate a progressive spring. Add on top a rather heavy rear wheel and an upright riding position to make things worse.

When I was riding the Livewire last year I was directly thinking of replacing the Michelin Scorcher Sport tires in favor of a tire with better damping properties (I ordered a set Pirelli Rosso IV for mine, but I was told that the Metzeler M9RR may have even better damping properties.) and increasing preload of the shock while reducing compression damping a bit as an attempt of a first simple fix.
I have not yet found an aftermarket shock specifically offered for the Livewire - the stock „Showa Balance free rear cushion lite“ item seems to be the same (except for the color) as that of a (>190 hp) 2020 Suzuki GSX-R 1000 R (see picture), i.e., it might indeed be hard to find an essentially higher quality aftermarket item. I have checked with Öhlins, Bitubo and Wilbers - these companies are not offering anything for the Livewire.

Mechanical engineering courses tell us that reducing unsprung mass might also be option for getting better ride comfort, but weight reduced BST Rapid Tec carbon rims do cost a small fortune and then carbon wheels are not for all street environments.

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[Krylov]

Getting nonlinear springs is a bit of a delicate affair, as you need to get the spring rates involved right to get the desired effects. Getting a spring suitable to your own personal weight, however, is a sensible suggestion anyway, as the OEM spring rates are chosen as a compromise over all possible driver persons, mostly with concern for stability and safety at all costs, i.e., comfort issues may take a second place. (A 60 kg person will require a different spring than a 100 kg person to get optimal results.)
Playing around with damping settings (e.g. softening up the compression damping for a more comfortable ride) eventially is also a bit of a problem, as a softer setup may allow for more uncontrolled wheel movement and can result in a wallowy, unstable ride - not something you want on a bike capable of severe speeds at high lean angles.

As the Showa shock allows adjustability, It makes sense to first try and get the sag settings right by changing the preload of the shock according to your weight and then gradually reducing the compression (and evtl. the tension) damping, maybe starting from a middle position of the damping settings. (This is what these guys did.) There is a bit of a compromise between comfort and stability.
If a suitable or satisfactory setting can not be found, any additional change in the shock (such as the suggested change of the spring rates) might require contacting a professional suspension specialist (My own basics in mechanical engineering university courses are 30 years old by now...); this might also be the case if a replacement shock is in order as there are no big brand alternatives (Öhlins, Bitubo, Foxx, Wilbers,…) readily available for the Livewire.
I have found out, however, that this German company has build a shock for the Livewire for two Jens vom Brauck Livewire custom builds.

 

[xophere]

Yeah I would only do the springs and deviate from the recommended settings with the help of a tuner.

 

[Wavshrdr]

The problem with a progressive spring is the dampening most accommodate it as well. I did set the sag as best I could for my weight. Dialed back both compression and rebound to softest settings and started from there. The rear shock just punishes the crap out of me on any sort of sharp edged bump and doesn't have a lot of travel anyway. Dialing up the compression dampening just made the initial impact even worse (as expected). The rear rebound was never so soft ever cause any pogoing in turns. It is basically a POS and I have no idea of who and what riding conditions it was made for other than maybe just at a dragstrip that was smooth.

 

[Krylov]

I have given this some thought and I do not know, if all of my ideas are helpful to your problem, but please see for yourself:

Are you still running the stock seat? This is notoriously hard and with little padding. I find it to provide a direct connection to the machine (which occasionally is a good thing during spirited riding), but also to be rather uncomfortable, especially on longer rides. A little more padding might also reduce some effects of the impacts of the suspension. Did you consider going for a Corbin seat which looks as it could provide more padding (and damping)?

What size are you? I am about 177 cm which puts me in a slightly leaned forward position on the Livewire. Potential jabs from the rears suspension only partially go up my spine as some of my weight is put forward to the bars. If I were a taller person with longer arms I might sit more upright and get these impacts immediately in my spine, in which case other bars might be an option to change the „rider triangle“, i.e. the weight distribution.

Another idea: Are you still running the stock Michelin Scorcher Sport tires? While these may last for tens of thousands of miles (and are possibly designed to even outlast the heat death of the universe) I found them to be quite uncomfortable and giving insufficient feedback. I had them replaced by tires which should provide better grip (my first priority) and also more comfortable damping properties (I hope - I just had them installed and could test them myself yet.)
At what pressure are you running your tires? Maybe a small reduction of the pressure could already might help alleviate some of the damping problems you have described. As you already „played“ with the shock, however, I guess that you already have also considered this approach.
(When I was browsing through several tests of sport tires I found that the Metzeler M9RR sports tires were considered to have excellent initial damping.)

But maybe the problem is even more of a principle one given the small weight of the sporty Livewire (a lightweight bike in comparison to other Harleys.) and eventually your own weight:
Did you face similar problems on other H-D machines with direct linkage, say on softtail frame machines? The ratio of unsprung weight of the rear wheel and swingarm to the mass of the sprung frame (plus driver) is here usually smaller than with the Livewire, which weighs at least about 50 kg less than an average softtail Harley. (Shaft drive BMWs that weigh about 250 kg or less are also known to react on sharp edge bumps just as you have described, since the weight of the mechanical shaft drive exceeds that of belt or chain driven rear suspensions by several kilogram. This is less of a problem on the heavier bikes or those used for two-person riding.) This problem may become even more pronounced, if the driver were a rather lightweight person.
If this is the core of the problem, I am not sure if another shock with better sensitivity (less initial friction) and less high speed compression damping would be a satisfactory solution for you - if there were any aftermarket shocks specifically designed for the Livewire available on the market at all.
Theory says, that the sprung mass of the vehicle body will face slower changes in motion (i.e., high horizontal accelerations) if the unsprung suspension weight is significantly smaller. (This is why almost all luxury sedans typically are rather heavy cars.)
Helpful might thus be running either a higher sprung mass (=vehicle body plus driver weight. Do the problems with sharp bumps also appear if you are running a pillion?) or a reduced unsprung weight, which amounts of running lighter sports tires (touring or touring sports tires typically weigh 0,5 - 1,5 kg more to provide a higher tread depth) or even going to lightweight carbon wheels, which would be a very expensive solution.)

These are just a few ideas for possible small improvements, but eventually a Livewire as a rather light, sporty roadster bike may not become as comfortable as say a Honda Goldwing even with the best of efforts.