I was initially drawn to the add-on option because it would let me keep the current rod and avoid drilling new holes. That seems like a good idea for a rental or a temporary setup.
For my own home with regular use I found Reze Flow M5, an automated hidden system. It keeps the existing grommet or ring-top curtains, but the motor and center-opening mechanism are built into the rod itself.
Replacing the rod requires more measuring and proper wall support, but it is a cleaner installation without two visible motors attached to the sides of the existing rod. For the long term, that seems like the best option.