Dont laugh but is a used 4040 RO membrane worth anything to you Guys that use Resin?
Mine has just been replaced as its giving 012 TDS on my setup.
Its cleaning OK with no spotting , but I don't use Resin. Might be a lot better than what you're using and could save you Resin wise.
Lasted 4 years of part time work on Soft Harrogate water, so not complaining about that. Works out about £60 a year.