I started my tank with mostly used Aquasoil only as a substrate. That was about 4 years ago, and the plants are still doing well. I have added a couple of bags of new Aquasoil during that time, just to have a deeper substrate. The Aquasoil has not broken down, is still discrete particles.
If you want to do a Walstad tank I strongly urge you to follow Diana Walstad's directions for setting up the tank. Don't pick and choose parts of her method and mix it with parts of other methods. We know that her method works, but we should by now have noticed that people who don't follow her advice do have problems at times. Based on that I would use backyard topsoil capped with pool filter sand. You can probably sell the used Aquasoil for enough to make it worth trying.