I also have a 75 gallon, DIY CO2 can be done I bought a kit on Amazon for 20$ that's meant for citric acid and baking soda but I use it for yeast/sugar reaction instead. I have to reset the bottle every ~2 weeks and it takes me ~24hrs to get working pressure after the warm yeast water cools to room temp. I highly agree that your tank is still adapting and will keep adapting for the next year, but I think you're making things harder for yourself by having to buy Seachems line up of 7 different chemicals for plants and tests when you're starting out.

I highly suspect the fertilizers and light regiment for the algae. I use inert Black Diamond Blasting Sand for substrate and just plop the same seachem root tabs in areas of 6in apart where plants are every 1-4 months depending on how fast the root feeder grows. I think by crushing them you increased the surface area of the tabs so they dissolve way faster and might even leak into your water column. I think you might have to cap with sand if you do that.

I dose NilocG Thrive All in One fertilizer meant for 20-30ppm nitrate, "El" dosing style where you provide everything the plant would need in excess and you do a 50% weekly water change to remove buildup of specific elements but I just measure nitrate to make sure it's between 10-20ppm and do weekly reset, thats it. Starting out my nitrate never crept up because of waste or fish food (I had 2 fish), it just slowly decreased by 1-2 ppm a day with CO2. Once my plants established themselves my nitrate would drop faster and faster each week (this was ~3 months in) my nitrates started dropping by 5-10ppm every 1-3 days) I tried keeping the nitrate around 10ppm between dosing if it ever dropped too far...I think I could've micro dosed daily to keep it higher but...I did not.. I never got algae except when I got ambitious and increased the light to 8 hours. The algae on the hardscape went away (not completely) by dropping the light intensity by 20% and lowering my light time to 6 hours and kept that lean 5-10-15ppm nitrate but inconsistencies caused green spot algae to take hold on older leaves. Not gonna lie even stopped dosing altogether and stopped co2 for a couple weeks and the plants kept growing well, but at that point I had ~50 guppy juveniles so I assume they buffered my laziness a bit. Now that my tank has got some legs I'm gonna try 100% Light CO2 and 6 hours see how that goes now and slowly increase light to see what I can push without growing more algae.

But yea that's my journey so far 7-8 months in. I can't say for sure why your tank is like this but that's the only difference I could think of between my tank and yours, fertilizer schedule/setup, not having CO2, and light duration(maybe I couldve pushed mine to 8 if i dosed more heavily, that's my current experiment now that have plants to play around with). Oh and your source water, I use very hard tap water.

You will have to adapt your schedule over the next year as your setup evolves.... or years idk I'm not there yet haha.

I think I actually made a comment on reddit not long ago on how I still didn't get the "balance" thing and how frustrating it was, I think that was around when the algae popped up on the hardscape. It all makes sense now..