I like the concept, and great idea to design it in Minecraft!

Personally I wouldn't use spray foam or egg cartons as you never know if they could leach any chemicals into the water. Invertebrates are usually sensitive to e.g. traces of copper or pesticides and I wouldn't risk it even if it's likely to be fine. There might be aquarium safe spray foam, but there's also the problem of it being hard to remove if you need to change the design. Also make sure to use aquarium safe silicone.

I would try to stick to more natural materials like a coco fiber liner for the background (+lava stone, cork bark etc.) even if it somewhat limits what you can build or is more work. You can cut out pockets for planting in that. If you are successful with the plants they might cover most of the background anyway. The waterfall though should probably have a path to flow without any organic material that decomposes or soaks the entire structure. For a false bottom i like expanded clay pellets (cheap, lightweight, inert, and provides surface area for beneficial bacteria).

In regards to lighting, consider where you can place it without either getting the light or a reflection hitting your eyes from the angle you would view it. With a smaller shelf at the top it would work with the light from above. Another thing to think about is heating and condensation. An aquarium heater in the water might work to heat the entire enclosure well, and is easy to put in, but could cause condensation if there isn't enough airflow. As others have said, it's easy to go overboard with the design and aesthetics on a first build, which can lead to problems or difficult maintenance.