this is not a mean evaluation this is just another way of showing you the problem. my college tank was like this, what you describe, to a T.
always ask yourself what have you done exceptional, in any tank build. something needs to be exceptional, not just avg, then a chain of events will set off that works in predictable ways to get good plant growth.
you have an inert substrate, main problem. inserted fertilizers feed algae because your substrate which is inert otherwise is large grain, and there is quite a lot of exchange with the water table where algae are
the lighting system is barely sufficient for the kind of growth you want, if other factors were maximized but they arent.
the params you posted are clean water, low nutrient, but thats off API kits so even the values are just broad range indications.
you know how you can go into a wal mart and they'll have some aquatic plants in the tanks that look great? huge, green, no bad parts etc. well they are still slowly dying, and thats what has to happen now because those vigorous plants grown elsewhere are now massively downgraded in habitat and someone has to pay the price in biomass. knowing this allows you to predict, and change course.
your goal is to arrest and reverse that process. you have about three more months till they melt
forget homemade co2
go buy a co2 solenoid kit right now
and when you are tired of algae buy a UV
regarding algae from now until you've been keeping planted tanks for twenty years:
hand remove any algae as you research what caused it and its scientific name. ID has nothing to do with an algae problem. nutrients have nothing to do with an algae problem (overuse, missing nutrients, any form of nutrient balance) clean up crews have nothing to do with an algae problem. failure to remove the algae is the problem. those other things are prevention tactics.
so now that your tank is forever free of algae no matter what you do to it, try and maximize the CO2 and after that the lighting. substrate will get loaded in time. the only difference between your substrate and mine on my tank right now with no plant problems is mine has 12 yrs of guppy poop in the gravel.
dont try a low tech tank on the type of gravel you have without doing something exceptional. upgrade to metal halide lighting if you want to do that. lots of plant problems go away when you buy a vette of a light. I realize most do LED nowadays, im only recommending one for sure way of many. mh is bad awesome I know how 90s that sounds. it allows me to get away with other forms of neglect I enjoy avoiding. its such a beast of a metabolism driver in both corals and plants.
since you used inert gravel, and are pretty much limited to water table growth support, you want plants to be faster than algae so now you have to gas them. do now what took me 3 yrs to do and look back on
buy the dang co2
get used to going to welding shops once every three months forever. just get on it now and dont forget a UV sterilizer. I realize these are optional equipment, again Im just trying to save you three years hard work only to wind up doing exactly this heh