What about a three seat bench coming as the base model. If converting to an SUV, the bench would come out and go in the back. Buckets would then come with the SUV kit and go in the front.
Or just mounting points for a third seatbelt up front in the frame. Aftermarket can solve the issue with...
The frame does not look like it would accommodate rear doors in its current form from the photos in this article. One could hope. But price point is a major squeeze on this.
https://www.slateforums.com/the-engineering-behind-slate-truck-chassis-body-battery-safety-suspension/