There is a relationship between CO2 and hardness, which is often forgotten. Many aquatic plants can use bicarbonate ions as their source of carbon, rather than CO2. So if you have high carbonate hardness in your water, additional CO2 would probably make little difference, whereas if you have soft water it would probably be more significant. I wonder what Easycarbo contains, maybe just bicarbonate??
Anyway, my point was that I have some tanks of similar size to the one you mention, with hard water and a full load of fish, plus an under gravel soil substrate, and the plants grow like crazy without any additional CO2. Rather than trying to make them grow better, I have to prune and remove some regularly. In my case, additional CO2 would be a waste of money and effort since my plants grow very well without it.
The only time I have found that the plants didn't grow so well was when I had just a few fish in a tank. Fish produce plenty of CO2 which should be enough in a normally stocked tank. When I increased the stocking to the normal level, the plants really took off. However, most aquascaped tanks have a lot of plants and very few fish, which is no doubt why they find it necessary to use some sort of CO2 generator.
I was given one of those CO2 generators which use a sugar and yeast mixture, but it required very regular emptying and refilling and I really couldn't recommend anything of that type.
You also mention a gravel substrate. Many people find that plants don't grow well in plain gravel and I'd certainly start by using fertiliser root tablets first. There is always one limiting factor to plant growth and if they are short of one or more nutrients, they won't grow any better however much extra CO2 is added. The same principle applies to light, increasing the intensity or duration of light will not make the plants grow any better unless light is the limiting factor. The concept of limiting factors is based on Liebig's Law of the Minimum, which states that growth is controlled not by the total amount of resources available, but by the scarcest resource. So you need to find out which is the scarcest resource, if it isn't CO2 then it is a waste of time increasing it.