Completely changed the layout. I gave up on the red claw crabs and ordered vampire crabs. Now they're waiting for me at the shop.
They're more terrestrial than red claw crabs, but still amphibious, and freshwater rather than brackish. I removed all the sand and rebuild the stone wall further to the right, leaving less room for the water. The water level is also a bit lower. Behind the stone wall there is a layer of expanded clay pellets to improve drainage, and over that a layer of a mix of sand and compost. Last layer just compost. The soil is kept separate from the clay pellets by a plastic mesh between them.
The clay pellets should keep the soil humid through capillarity, drawing the water up, without making the soil soggy (although in the pics it does look soggy)
The planned plants are java fern, Java moss and cryptos. Possibly also an anubias, I'll have to see what the shop has available.
If I can find a micro orchid that could be an interesting addition.
There's a lot of condensation on the glass, and that should mean that humidity is nice and high: good for the crabs and good for the plants. Tomorrow I'll see if I can get a hygrometer to keep it under control.
The air temperature is 24 celsius, which is coming from an IR bulb. Water's too low to use a water heater.
A big advantage of vampire crabs over red claw ones is that they're easy to breed. In a manner of speaking they're like freshwater shrimps: the female carries the eggs and when they hatch she releases tiny little crabs that are immediately independent.
They are mainly hunting carnivores, so I guess I'll have to get some of those small crickets that are fed to reptiles for them to hunt, and they will probably also appreciate snails harvested from my FW aquariums, where they breed very easily as long as there's food. Could be a good way to also provide calcium in their diet.
They are omnivores though, and they will eat anything they can find. They will prefer by long to eat meat, but will settle. Plants should be safe as long as there's other food available, and they should settle easily on a diet of flake and pellets as a staple.