Your substrate doesn't need to be flat 😉 you just need to get creative with containers and capping.I've done 3 dirt tanks, but will never ever do one again. Dirt's only advantage is cost of setup. The dirt is literally free if you dig it up in your backyard and you probably should since its almost certainly better then anything we can buy in stores. But you can make the same scape with aquasoil you do with dirt and aquasoil will only run you around 20 to 50 dollars to replicate the same thing you are doing with the dirt. And aquasoil doesn't cloud the water, or have unexpected crazy algae blooms, or in general make a giant mess every time you go to move a plant. You can also rescape substantially easier and have substrate that isn't just flat. So saving that 20 to 50 dollars isn't worth it to me the headaches down the line.
Do you still have the 20 gal running? Would love to see it.I've got quite a bit of experience with ntp/ walstad. Greatest experiences- small to moderate (5- 55 gallons) sized aquariums and vases. Larger displays are harder because they require a powerhead arrangement to get flow going and keep from getting oil slick water with no o2 exchange. My strongest suggestions are to keep your scape simple, use only hardy low light plants. Keep an eye on your parameters during start up a little more than you normally would as a leaky cap is a recipe for a tear down and redo. I strongly suggest leaving your scape alone once it's planted to avoid excess nutrients getting into the water column and causing an algae bloom. Make sure your tank is fully cycled and has matured enough that your livestock won't be able to tear it up and finally, stock light to start and avoid anything that burrows in the substrate.
My favourite set up was a 20 gallon heated walstad betta tank. Kept it going for years with just top offs and water changes a couple times a year when I'd prune the plants. Had a desk lamp with a compact florescent for supplemental lighting, no co2, no ferts. Though my plants list was basic- windelov java, narrow leaf java, crypt lucens, crypt wendtii, crypt parva, red nymphea, aponongeton ulvaceus and crispus, java moss, marimo, anubias nana petite, anubias nana, anubias coffeeofolia (or however you spell it) and floating hornwort.
Worst experience- 30 gallon where I used potting soil that I bought instead of my usual baked dirt. I didn't sift good enough and some perlite and wood made it in. Cap failed when I had to leave town because of a family emergency, algae took over by the time I made it home 2 weeks later. It was too far gone. I had to tear it down, scrub everything and start over. Luckily there was no livestock in the tank yet.
Sorry for the long read.