Your numbers are looking better each time so you're getting close. Keep at it and you'll be done soon. You may see it stall for a little while but don't lose faith.
Changing that old cartridge is perhaps an idea worth entertaining. I'm not a cartridge expert so wait 'til some others chime in but I could see how it might speed the process a little and finally get you where you need to be.
Cha-ching!!! That's the whole purpose!The pool water is crystal clear, blue, and sparkles!!!