If you have natural dirt topsoil in your area, skip using the potting soil and actually go out someplace to get the dirt.
I've found that a thin layer of fine gravel placed on top the dirt makes a huge difference in the management of the tank.
Too many plants? Well, the Walstad is based on having enough plants to support the fish, so if you have tons of fish in your plans you need tons of plants. The easiest way to plan this is to get several large plants and many many small plants then plant them all. Java fern like much rockier soil than dirt and it would not be un-healthy to have a small rocky area someplace in the tank to accommodate such a hardy plant.
The method I used in my boosted walstad tank was to spread out the dirt about half an inch thick then stir in actual aquarium water from an existing tank. Then add more layers of dirt like cheese on a pizza while adding just enough sprinkled water on to wet it all. Once I had one and a half inches of dirt I then leveled it out evenly by hand by patting and smoothing.
I then put on a three quarter inch layer of wet black flourite (expensive) to keep turbulence from being a problem and moved the tank to where it was going to spend its life. Be careful if you assemble the mud bottom before moving, they get very very heavy and you'll need gloves to protect your hands.
Once I had it where it was going to go I added four inches of water and stuck a wooden wand much the size of a drummer's stick down in the dirt and patted the side of it for every square inch. Bubbles in tons, live bacteria.
Cycling a walstad is painful, the dirt has its own ideas as to what the PH should be and changes its mind every day or so, large plants won't help any more than properly planted small plants but you want to use actual rooted plants or root-bulb plants. Onion and Lilies are great, so are Crypto, Wendtii and some others. I've had no luck with Cardinal but other soft broadleaf plants have done great in it.
I have a Crypto with a ten inch root reach after one year.
If you use actual yard topsoil instead of potting soil you avoid a great deal of the troubles normally had with potting soil; decay, peat, additives they lie about, animal wastes, litter and debris. Some places actually decay road kill inside their mulching piles to produce the finished product...
Before adding fish but after the dust has settled out well you want to run carbon filtration for one week when you use yard soil if you're anywhere near industry.