Hi Caillou!
With those mods it looks like you have the low-hanging fruit.
I saw some specs somewhere that your model should use an average of 154Wh/km. Have you been tracking energy consumption before and after the mods?
And yes - smooth underside and especially a short Kammback would give you more savings.
I assume that with an electric cargo minivan with a range of 200-300km you probably tend to drive in the city more than on expressways, but still the Kammback would give a noticeable improvement even at speeds up to 50 km/h.
The issue you might have would be legalizing the Kammback if it were fixed to the car itself, but you could try to make a hitch-mounted Kamm, like Theo22 did on an Audi A2:
Threads for this:
DIY hitch-mounted boat tail on Audi A2 3L (photos)
My Aerotail on Audi A2
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e·co·mod·ding: the art of turning vehicles into what they should be
What matters is
where you're going, not
how fast.
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[Old] Piwoslaw's Peugeot 307sw modding thread