Powder coating, which you already have, is one of the best surface protections you can get for metal and ali surfaces. You could add a two pack clear varnish but these tend to yellow with time and you may already have some form of clear top coat.
I looked at a low salt SWCG variant, they were 2-3X the cost of a standard SWG. As mentioned both bleach and acid add salt to the water. I’m now just on a month after startup and we’ve gone from ~120ppm salt to ~750ppm from bleach and acid additions.
While aluminium would not be my choice of material for pool coping you have to work with what you have. At ~4000ppm the salt level for a SWG pool is around 10% that of a marine environment at 35,000ppm and now where near as harsh. Having said that powder coated aluminium is very common for pool fencing and gates etc. and your coping should give you years of service, just be cautious of both acid and bleach around the pool.