I have a BEV (totally electric).
I suppose that thos i3 owners do that in order to get better battery life than lead-acid. But I'm not worried about that, the car is fairly new.
I'm just looking to extend the range a bit using solar. I'm keeping the 12v lead-acid (SLA) because I'm trying not to make any permanent modifications, and it won't help extend the range anyways.
Replacing the 12v lead acid battery with LFP increases the range because it decreases the weight of the vehicle.
Adding solar panels, a charge controller, and an additional battery decreases the range because it increases the weight of the vehicle.
If range is the goal, you really should measure the amount by which your range changes for every kg added or removed. You'll have to get enough solar input to overcome this loss. Also aerodynamic loss if you're adding things to the outside of the car.
If playing around is the goal, then continue enjoying yourself.