I agree with you, there is no magical answer that cures all deficiencies. The "natural" reaction to a plant that fails to thrive and produce in the conditions readily available, then let it die and replace with something that can. Many of the fruit trees we plant are non-native and have no business being there anyway.
As far as what nutrients a specific plant can add to your garden, my own belief is that certain plants attract certain wildlife. Every insect, every worm, every snake, every bird generates waste, sheds skin, dies, etc and that biological material is also added to the soil as it breaks down. At the very least, a well-developed garden attracts abundant wildlife that can bring in additional nutrients beyond what the plants contribute.
I've been thinking about this comment lately. Doesn't it also work the opposite direction too? Birds for example. Cattle egrets more specifically is something I observed that made me remember this comment.. When I see them visit our cows in their silvopasture setting they usually show up about an hour after dawn, and descend into the paddock with the cows, stay for most of the day and then fly away in the late afternoon. They nest somewhere far away (not on our land) and it's great while they're here pooping, but they also eat bugs and whatnot from our property, and I assume a portion of those nutrients are then exported after they fly away.
Or leaf cutter ants that are on a neighbor's degraded property and walk a half kilometer to defoliate my fruit trees and carry the vegetation away into their underground nest that's also not on our land.
In my opinion, it is rare that nature itself is really *stealing* from your land in a meaningful way. It's usually pretty close to net zero, even in the case of egrets eating things and flying off, or leaf cutter ants taking leaves. Even with gophers in CA, which are the bane of my fruit tree growing existence, they can be reasonably helpful to loosen up hardened clay soils and may have a purpose. It's just the reality of your environment and you'll need to find a way to work with them, or deter them.
In any case, it really does sound to me like you just need to find any organic amendments you can acquire easily and cheaply in your area. It can be anything. Wood, manure, leaves, coffee grounds, agricultural by products, etc. Do they do any sugarcane processing near you? The squished canes are nice fluffy wood. Is there a sawmill nearby? That has a ton of great fluffy sawdust they'd love you to cart off. Think of those types of industries and see what you can do.
