Excel is great for low light situations, but it can be toxic to fish if overdosed. I don't think you can use enough Excel safely to match the light level you have already.
The planted aquarium is all about balance (and patience!).
Light, carbon, ferts - the holy trinity of planted aquariums.
The thing about the DIY CO2 is it is hard to control. I really don't know if 1/2 liter is enough or too much for a 5.5G tank. You won't know until you set it up, fix all leaks and test the water to see how low the pH goes and therefore how high the CO2 is. The recipe you use, the volume and freshness of the yeast, the type of yeast, the room temperature, the length and composition of the airline you use, the porosity of the airstone you use and if or when it gums up if you don't have a scrubber inline, and your tank's water parameters, filtering system and circulation patterns... all these variables affect the CO2 you will get in your aquarium with DIY, compared to what I get in mine with DIY.
For that reason, I suggest the Hagen system. Most of those variables are controlled, and you have a built in means to adjust the CO2 diffused in the water by raising or lowering the level of the input line. You can easily choose 100%, 75%, or 40% of max output. I have no way of slowing down my DIY output short of dumping the container and remixing it with less sugar and a guess as to what the rate might be initially. With a 29 gallon tank, this is not a big issue, with a 5 gallon tank it might mean the difference between fish and no fish some morning.
For $16.50 plus shipping, it is a no-brainer. Get the Hagen. While you are at it, buy the testing kits you need, you will save so much buying online that the shipping cost is absorbed and you still come out ahead. The more you buy, the more you save on things you need to get anyhow. Ferts for example, you will need some.
Heck, get two Hagens, so you can plant the 10 gallon and then you'll have enough spare CO2 resistant tubing to make a good DIY for the 20 gallon after you get rid of the goldfish and turn it into a planted tropical tank.
If you just like to tinker, you'll have only 3 packets of mix in the kit, then you'll need to either buy more or start making a mix yourself. Let's see, beer yeast, champagne yeast or bread yeast? Sugar or gelatine? Baking soda? Yeast nutrient? Water temp? Sterilization or save the yeast cultures?
And anyway, once you get the CO2 going and stable, you will really, really need to know about ferts with that light level.
It certainly is possible to overdose a tank, even with the Hagen, and certainly with a 2 liter bottle on a 5.5 gallon tank.
I've read of people who nearly OD'd a 10 gallon with a Hagen. You probably couldn't run it at the lowest setting with that small of a tank. It reallly depends on your water.
It is unwise to begin with CO2 until you know your starting pH and KH. Then you can establish what your target pH will be (about 0.8 to 1.0 lower generally), and the lower limit of pH will be, below that you will risk killing your fish. All this comes from the CO2 charts. And your water parameters.
So, what is your starting pH and KH?
FWIW, The hagen system is clean and neat and hangs on the back of the tank, and it fits the 5.5 gallon tank. All the needed parts are included. The unit does not leak, since it hangs on the tank it will not siphon water if you remove the top to refill. The mix is very good but you can run your own mix when it runs out. The diffuser never plugs up like an airstone can. After 4 airstones dissolved in my DIY setup due to the carbonic acids I gave up. The airstones also gummed up which required a bubble counter to scrub the goo out of the gas. All those connections leaked initially and were a bit of a chore to get sealed, even now I have to occasionally scrape the DIY set clean and reseal. The smaller tanks both have Hagen systems.