First things first, if you bought the kit that uses little tiny CO2 cartridges for CO2 injection, return it. It's relatively economical to start up, but you could buy a new car with the amount you'd spend on refills for a tank that large. :-) Just in case.
is a good introduction to the interplay between pH, dissolved carbon dioxide, hardness, and KH (aka alkalinity.) It's intended for aquaculturists, but the chemistry is the same for an aquarium.
It looks like you have a pretty good understand of the interplay between CO2, pH, and KH, however, the value you've come up with isn't the maximum capacity of the water, it's the equilibrium value for the concentration of CO2 given our atmospheric conditions. You can force far more CO2 into solution and it will tend to outgas to the atmosphere.
There is a lot of sloppiness in the terms people use, hardness, carbonate hardness, GH, KH, etc. I've been reading a lot to try to pick out the meanings of these things and honestly, you see a lot of cockeyed advice on this forum and others. They can be strange ideas to begin with, and their interactions are complicated.
Hardness (AKA GH: concentration of ions with charges greater than +1, typically calcium and magnesium but iron is another common one) has little to nothing to do with your pH in and of itself. It's a measurement designed to let people know how much soap to use that has been adopted as a convenient value for a host of other things, whether it is appropriate or not. It's important that water not be extremely soft because fish and shrimp require some level of dissolved calcium and magnesium for metabolic processes. The multivalent ions are technically weakly acidic, and they also help to provide buffering against additional bases added to solution. These aren't very important to us usually because of the vastly stronger carbonate buffering that is also occurring, and for practical purposes can be ignored.
KH (AKA carbonate hardness: concentration of carbonate and bicarbonate in water, closely related and often synonymous with alkalinity, but alkalinity also includes concentration of hydroxides and borates) is a measurement of the buffering capacity of your water. As KH rises, pH also rises, but fluctuations in pH decrease. Water with a high KH has very very stable pH values, but that pH is typically somewhat high. Higher KH means that additional CO2 is dissolved in water at a given pH when it is at equilibrium with the atmosphere (i.e. less outgassing occurs at a given pH.) That's rarely a consideration for us.
pH (the opposite of the base 10 logarithm of the concentration of hydrogen ions in solution) is important to some species of fish and inverts, but for the vast majority of species, the stability of the pH over time is more important (as long as the pH is within reasonable bounds, of course.) As long as you have a reasonable alkalinity (buffering capacity) you should be fine.
The reason that so many of these things get confused and conflated is that one typical source of all of them is calcium carbonate (limestone) with calcium magnesium carbonate (dolomite) being another. When water comes from a limestone or dolomite aquifer, it tends to have high GH, high KH, and high pH. The three values are not necessarily linked though, at least to the extent that many people believe. The water from my well, for instance, has a pH of 8, a GH of 2.5, and a KH of 14. It's also entirely possible to have a very low KH and a very high GH.
In short, and to actually answer the question you originally asked, your water parameters look fine to me and you shouldn't have to worry about CO2 injection. The 9ppm value you found is related to the equilibrium concentration that your water would have with the atmosphere, not the amount that can be dissolved in your tank. If you find that your pH is moving around significantly from day to night you can try to increase your KH but you probably won't need to.