I asked about the lighting because it is the amount of light you have that drives almost everything else in the tank. You have high light, which means the plants need all of the nutrients, in non-limiting amounts, and good CO2 concentration. You aren't dosing any phosphates. When you don't dose phosphates, plant growth drops way down, and the plants are unhealthy. You don't have any CO2 to speak of - one DIY bottle is essentially no CO2 for that size tank. So, the plants have to find carbon somewhere else, and with that much light, they probably wont find nearly enough. Again, unhealthy plants, growing slowly, but with high light trying to force fast growth.
GH test kits, specifically the API kits, are not dependable. They may work well initially, but also may never work at all after that. I have had 3 of those, and every one failed to work very soon after I bought it. The failure was just as you describe - you add tens of drops and never get the color change. So, you very likely have normal GH.
A pH of 6.2 is normal, especially if there are some tannins in the water, or if you have good CO2 concentration, or if you have peat moss or other "softening" material somewhere in the system - the filter, the substrate, etc.
I think once you get a pressurized CO2 system set up, and use a drop checker to encourage you to raise the bubble rate high enough for it to give you reasonably good CO2 concentration, and start dosing some phosphates, like KH2PO4 or Fleet Enema, you will have healthy, fast growing plants, and you won't care what the pH, KH or GH are.