I am going to answer my own question. I recently installed 175/55r20 Ecopia 500 tires on the 430 front wheels and 195/50r20 tires on the 430 rear wheels of my 2017 i3 Rex with no spacers on any wheel. This is the standard tire set used for the current version of the BMW i3s Rex. The installation went smoothly, and there is sufficient clearance between the suspension parts and the wider tires all around. The ride now seems a little more stable, and I have not detected any appreciable loss in the vehicle's range. I am running the front tires at 38 lbs and the rear tires at 46 lbs to reduce rolling resistance.
2017 BMW i3 Rex