What I find most interesting is the possibility of replacing the seat with a custom fiberglass seat pan to cover the electronics, as the seat itself makes up 3-4" of additional height. Reduce that down to 1.5" and you'll get 2.5" lower. I think the additional height comes from the seat shell itself, not the padding, but you could also try reducing the padding. Basically, there is a bunch of air space under the seat pan, at least on my Zero S.
The easy things to do: reduce your rear preload to zero and slightly lower (1 cm) the fork tubes on the triple tree. Next you could go to lower profile tires. Then you next step might be modifying your rear shock to reduce it's total length. I did bring my Zero S into a suspension shop and although he did say the rear could be lowered, his first inclination would be to try to shave the seat down.
I see the DS has a 19" front wheel, but I imagine swapping that out for a smaller wheel may be complicated because of the brake caliper.