Even though you have it more under control, I wanted to just add a few things...
I have kept planted tanks for years.... I have tried many plants, setups, and substrates.
A couple of things everyone should remember:
You said once you gave up, things started surviving. Not to "blame" you or otherwise, but one thing I noticed right off was you were chasing your tail. It is like the new fish keeper who says " I don't know what the heck to do. My fish keep dying. I just started my tank, and I put in x fish and everything was fine. Then some died. So I changed my water 12 times a day, siphoned the gravel, changed the filter medium, turned the light off, took out the plants, etc... etc..."
When you do such things, you have no 'base'. You need to STOP. DOING everything is creating part of the problem.
So, changing 50% of the water twice a week is a bit of the problem. If the plants you wish to grow are "rooted" then you siphoning the gravel like crazy doesn't help either.
Nitrates are not great things to have over 40-50 ppm. But zero sucks too, for plants.
Remember, healthy plants in adequate numbers will always out compete algae (as long as other things are not grossly out of balance due to poor fish keeping habits, like over feeding).
Remember also, a well planted tank with adequate filtration and a bio-load tha is not grossly out of whack, the plants will do a outstanding job of keeping the water in balance, i.e., ammonia and nitrates. 50% water changes with Discus sounds good, but you are not considering the needs of the plants, and their ability to keep the fish "good enough" for Discus.
Planted tanks that are beautiful are a balancing act. Light- duration, intensity, and quality (kelvin/spectrum) as WELL as lamp health ALL play into the factors that light can affect.
This is balanced with nutrients- ammonia (waste) and nitrates, substrate elements- as well as with CO2 and micronutrients. I would ALWAYS tweak CO2 and micronutrients LAST. NEVER first. I would not even consider USING CO2 until I had a balance I could trust, and then I would add it slowly and scientifically while at the same time adjusting light ever so slightly upwards.
I have used CO2 injection in the past. I think it is an overrated system of "help me grow plants better because I am too impatient to balance the other things properly first". If you do EVERYTHING else well, the plants will THRIVE. After all, they do in nature, just fine, without CO2 tanks.
I used CO2 for a few years. I also almost kill every fish in my tank (probably about 65 of them at that time) when the regulator freaked on me. Ever seen a tank full of fish freakin on CO2? If you ever do, you won't forget it. That tank got turned off and has not been used since. (By the way, I used a bubble counter, and infused the CO2 in to the column in a Magnum 350 via the impellers... great system. However, I never saw results that made the risk make sense, let alone cost.)
The key is balance, and you can never find that changing everything in a frantic mode of "holy crap!"
Personally, if I was to give advice to people on this, I would say "Buy intense lighting, 2-2.5 WPG minimum; no UGF's; moderate population of fish for the tank (keeping cycle and nitrates up); water changes at a LOWER rate than normal for such a population; buy the plans you want to keep according to depth and substrate conditions; if you get algae problems either step lighting back slowly (.5 hour a day) and wait for a couple of weeks, while at the same time adding some floating plants that propogate fast like frog weed/duck weed/ water lettuce, and just take it slow and easy.
Once it balances out for plant life (low to negligible algae growth) then, you can start trying to add more plants. Then, when they don't grow well, (IF they don't grow well- you might be surprised...) you can screw with CO2 or lighting increases or both...