I take it that is a DI vessel on top of the tank?
Is that wheelie bin in use at all?
To protect your RO membrane, get another DI vessel and fit it in the hose line before your 2 prefilters. Fill it with water softening resin.
Also get a clear plastic 10" salt vessel with hose in and hose out with basic hoselocks on. This is just bolted to the wall and is independant of the filter system.
After every tank fill, put regeneration salt in the plastic vessel, connect your hose pipe to the inlet side of this vessel and connect the outlet hose of the vessel to the inlet on your softerner vessel...from the outlet on your softerner vessel, connect a length of hose and run to drain.
Turn tap on and the salt is dissolved into the water stream and runs through your softerner vessel removing all the calcium and magnesium that the softerner takes out the water...when the salt vessel has clear water in it, turn off tap and disconnect hoses...reconnecting the hose line to the inlet of your now regenerated softerner vessel.
This ensures all calcium and magnesium are removed from the water before they get near your pre filters and more importantly, your RO membrane.