I also use a failry thin layer of black gravel in my 3.5 tank - and I have to thin the plants out every now and then these days, or there'd be no room for the fish! I use no cO2, no ferts. Just fish poo and a good LED light which supports plant growth - all the plants are 'low light' easy care types - java moss, java fern, a couple of crypts, anubias..
The crypts and anubias are healthy but growing very slowly (in this tank, this is a good thing..) but the moss, susswassertang, java fern and banana lily are going nuts. My wisteria does better floating than rooted, as I don't use root tabs in that small tank, or add the iron it craves - it goes pale after a few months, and I move it to my NPT where it thrives.
My pH is about the same as yours.. I have no
idea why yours are failing so dramatically! All I can think is: you need better lights, maybe try plants with less roots, cutting the co2 for a few weeks, and do a drop of ferts a couple of times a week.
Is your java fern in the substrate or tied to something? It hates being planted, as the rhizome rots away.