Thats a lot of amazon swords for a 30 litre tank and even 1 sword would probably outgrow your tank. It does look like normal plant melt which is what happens when a plant transitions to fully submerged growth. You should have started to see some new growth after a couple months. Some root tabs and liquid fertiliser might have helped.
Java fern and anubias would have been great plants to start with. They just need attaching to a piece of rock or driftwood and placing on the bottom. They dont need to root into the substrate and are super easy to keep. Marimo moss balls are easy too. Moss balls are great first pets for kids keeping them in a jar.
Bigger gravel will be more difficult to keep than smaller gravel. Dirt and waste can more easily get trapped in the bigger voids between larger pieces of gravel, needing more cleaning to prevent it decomposing. The gravel you have looks too big IMO
. Smaller gravel or sand will keep waste on the surface and be easier to clean.
You will need to recycle your tank, but you've done that before.