The two feeding styles I'm familiar with are topping up with straight water until a drop in ppm low enough and do a complete change. Or top off the res with pre mixed nutrient solution until the solution gets too strong and change out then.
I've never had an issue topping off with RO water if my res has sufficient nutrients for the size of plants in the system. Something like 1-2 gal per medium to large sized plants. Your system seems more than large enough. Any foul smells from the res or roots?