If you're driving an i3 in a warm environment and your legislative environment allows, step one would be to tint as many windows as you can, as darkly as you can. Aircon is a major power draw for any EV and the i3 has high IR gain from its huge windshield.
In terms of driving, I haven't had the opportunity to wring out a 2018+ i3s so maybe BMW has solved things at the factory by now, but...
From the perspective of a 2014 owner interested in a more driver-oriented ride, changing to stiffer springs that also lower the ride height makes a big
difference. An early production i3 has a lot of body roll so stiffer springs do help here.
In late 2014 the only company that made springs for the i3 was Cobra in The Netherlands:https://www.cobra-suspension.com/
Now there are many companies that do so, including favorites like H&R.
If you have a good friend that lives outside the USA, here are two convenience items that are not available from a BMW NA dealer...
82 29 2 151 462 On-board toolkit
71 60 6 770 487 Safety triangle and case (includes clips to attach to the inside of the tailgate)