I don't see much of an algae issue in that tank. I mean you cannot expect a tank to be 100% free of algae that's just not possible. Your tank looks just fine in my opinion.
As for the PH/KH chart, that chart is theoretically pretty accurate for degassed water as long as there is no other contributors to PH other than carbonates and that's the issue in our tanks because we add soil, woods, ferts etc etc which buffer the water. So you end up with a lower PH before you even start adding CO2.
No CO2 testing method is perfect. DC or PH drop both have their downside. I prefer the PH drop test for multiple reasons specially because you don't have to wait several hours to have and idea where you CO2 is at. Pick the one that suits you most.