Firstly, I dont think well ever reach a stage of no algae although it would be nice.

Be careful of changing sand too often, Ive read that a common cause of brown algae is the silicates leeching from new sand/glass.. hense the problem is generally found in new tanks. Ive had this same problem, once Id left it alone, upped CO2, quit fluffing up the sand to hide what was on it, it went.

It also sounds like you have GSA on the glass. Alot here recommend letting the GSA complete its life cycle, then it dies, and you likely wont ever have it again.. I know thats worked for alot of people on the forum.

It does sound though like your CO2 just isnt enough, theres no way we can use the KH/PH method of measuring with any accuracy.. once you can buy a drop checker, that will make your life alot easier. You have high light, so your CO2 demands are likely to be very high indeed.

If your following EI properly, then you can rule out totally any macronutrient issues. Because of the small tank, any issues you get are going to be magnified because of the lower water volumn, if it were me, Id probably turn down the lighting a bit, to something like 2w/g for 10 hours a day.. this will slow everything down nicely and allow you to get on top of it. Especially because you cant be sure of your CO2 levels.