I had an auto top up on my old jbj nano. Definitly worth it as the return chamber is so small and you have to top up every day. I made a small 5 litre tank behind my tank as the reservoirs and then a float switch in the return chamber via a bracket i made out of perspex. I drilled a small hole in the back of the canopy for the pipe and wire of the float switch
I still prefer to have a separate topup tank since I like to flush my RO unit before using the RO. I discard the first 5 litres of RO the unit makes as this normally has a tds reading of more than 15. But then again my unit is very old and filters have not been changed in years. I know the newer units have an auto flush function but I still prefer to discard the first few litres of RO. And the more electronics you add to the system the more things can fail.
The RO unit also sometimes start leaking at the seams due to back pressure if I forgot to turn it off by tap and the physical float switch closes the outlet by the tank. Is there another way to switch it off after the RO unit without having the unit leak or the waste water to still run?