You need CYA...put it in. Either via tri-chlor pucks (which will allow you the luxury of not having to put bleach in every night), or by adding dry CYA directly to pool. The pucks have CYA and chlorine, you'll get both chemicals this way. However, the pucks are acidic and have the tendency to drive pH down. You'll need to monitor that.

My pool is in direct sunlight all day and has moderate bather loads every day. My CYA is currently at 70ppm. Chlorine holds at about 3-5ppm for approximately 2 full days. All things being equal and no extraneous variables (pool party, heavy rains) I add 3qts of bleach every other evening. My pool is fairly large: 24,000 gals. With larger pools you will go through a TON of bleach. You have a BIG pool. Without CYA, the chlorine will not hold in your pool. You will go through a lot of bleach...>$100/season.

SWG should pay for itself in a few seasons just on bleach costs alone. However, your time and convenience is also a cost. I'd get the SWG.

CaryB