How to efficiently charge 48V battery-motor system with solar panels
After the discussion in the comments above and asking some electrical engineers, I decided to go with the following setup:


leave the 48V battery-motor setup as it is
wire three 18V panels in parallel. While this is not optimal, the change in voltage due to shading or orientation should be much lower than the loss in current, so the total power output should be higher than wiring them in series
use a MPPT boost solar charge controller that will handle the panel output, boost the voltage to the level needed for battery charging and prevent overcharging of the battery


This is the low-cost version that will hopefully still yield good results. After construction and testing, I'll update you how well it works (or not).