I’d be REALLY surprised if there are ANY veggie meatballs that will tolerate a 3-hr simmer, regardless of sauce. Much better to make the meatballs separately, then hold them in warm, but not simmering, sauce. But if you are determined to try to achieve this, I would get some balsamic vinegar in to the mixture, because acid slows the softening of vegetal ingredients. If your constraints don’t preclude cheese, include grated and/or ricotta cheese, plus egg, all of which will contribute to a firmer ball. Farro would be a good starch component, because it has a meatlike chew.
I put a LOT of vegetables and other filler into my beef meatballs. Because of this, I use a mandoline to slice onion and bell pepper into very thin rings. As the mixture is handled, they break into strings that create a matrix to which the other ingredients cling, so the balls don’t fall apart while simmering.
All that said, I like Trader Joe’s frozen meatless meatballs, the flavor a9d texture of which could almost fool me into thinking they had a little meat in them.