Bioactive tank setup?

We own an Asian Bullfrog named Zeus. (Zeus is an unsexed adult but because of the name I'll refer to our frog as a male.) He currently lives in a 50 gallon tank. We are going to downsize to a 20 gallon because the frog uses only the water dish and the area directly outside the water dish with most of the enclosure being unutilized.

His current set up is a basic water dish, estimated to be about 2 liters. The water dish has a ramp to make hopping out onto land easier. It has about six inches of substrate, mostly coconut soil with about one inch of dried moss. There are a few pieces of artificial decor; a fake plant to hide the moisture gauge, and three tiny statues to make the enclosure look like a forgotten ruine. We have tried to keep varies live plants in the enclosure but haven't found any yet that will survive. We have a heat bulb on the side of the terrarium he doesn't use. I'm thinking of switching it for a heating mat for the downsize. For soil clean up the tank has worms and springtails. There is also an unintentional beetle colony, before we started using a feeding box we would release the feeders into the terrarium and a few survived. I'm kind of fine with that as they can't escape and I figured they can be a late night snack for him. We mist Zeus's terrarium as needed.

I'm thinking of switching Zeus to a 20 gallon. We would keep the two liter water dish, adding an air stone for water flow. We would add in some more worms and springtails (or isopods depending on what the local pet stores have in stock.) I'm thinking of adding some lava rocks or porous ceramic beads to the bottom substrate.

What plants can we use that might actually survive the mighty Zeus? Is there anything we are missing for his downsized terrarium? Pothos don't seem to work in his current enclosure btw.