I had a 10g, very heavily planted, lots cherry shrimp that was "almost" self sustaining. I fed it maybe once a week or 2, a pea, a food pellet or 2, or tiny pinch of flake. I'm not sure they needed that much.
I changed water, well, maybe a bit more than once a month but not much. If the shrimp were happy (& mine were), they bred often & a lot. I gave away or sold many...Plants too.
Even a plant only tank needs some help. I've read it's "theoretically" possible, but not in any real life way, depending on your water supply, plants & animals. Even just topping up water evaporation seems maybe like it needs more effort than you want to give...What are you trying to do? Habitat sustainability, or...economics... or what? What are your water parameters? That could go a long way to inform us of your tank's "sustainability" possibilities. Even Walstad tops up as far as I've read of her method...