Fluctuating CO2 levels can create conditions that favor algae over plants.
I'd get a drop checker with a calibrated 4 dkh solution to get a better handle on your CO2 levels. There are too many other variables that can affect pH levels besides just CO2.
When you list out all the things you've tried- how long did you try them all? It can very easily take weeks to months to get algae outbreaks under control. You need to take things slowly, change only one thing at a time, and thereby eliminate possibilities one by one.