結論

import SwiftUI

struct ContentView: View {
    @State private var scrollTrigger = ChangeTransmitter()
    var body: some View {
        VStack {
            MyScrollView(scrollTrigger: $scrollTrigger)
            Button("scroll to bottom") {
                scrollTrigger.trigger()
            }
        }
    }
}

struct MyScrollView: View {
    @Binding var scrollTrigger: ChangeTransmitter
    private let scrollTarget = "bottomID"

    var body: some View {
        ScrollViewReader { proxy in
            ScrollView {
                ForEach(0..<100) { index in
                    Text("\(index)")
                }
                Color.clear.frame(height: 1)
                    .id(scrollTarget)
            }
            .onChange(of: scrollTrigger) { _, _ in
                withAnimation {
                    proxy.scrollTo(scrollTarget)
                }
            }
        }
    }
}

struct ChangeTransmitter: Equatable {
    private var changeCount = 0
    static func == (lhs: Self, rhs: Self) -> Bool {
        lhs.changeCount == rhs.changeCount
    }
    mutating func trigger() {
        self.changeCount += 1
    }
}

#Preview {
    ContentView()
}

経緯

このように書いても…

import SwiftUI

struct ContentView: View {
    @State private var scrollTarget: Int?
    var body: some View {
        VStack {
            MyScrollView(scrollTarget: $scrollTarget)
            Button("scroll") {
                scrollTarget = 1
            }
        }
    }
}

struct MyScrollView: View {
    @Binding var scrollTarget: Int?
    var body: some View {
        ScrollViewReader { proxy in
            ScrollView {
                ForEach(0..<100) { index in
                    Text("\(index)")
                }
                Color.clear.frame(height: 1)
                    .id(1)
            }
            .onChange(of: scrollTarget) { _, newTarget in
                withAnimation {
                    proxy.scrollTo(newTarget)
                }
            }
        }
    }
}

#Preview {
    ContentView()
}

一度Scroll to Bottomした後、手動で位置を変え再度Scroll to Bottomを試みても、二回目以降は onChanged{} が呼ばれないためできない。

以下のように書いても、変化が早すぎるためかScrollされない。

...
Button("scroll") {
    scrollTarget = nil
    scrollTarget = 1
}
...

また、Scroll to Bottomを実現したいだけならscrollTargetはMyScrollView内部に隠蔽してしまいたい。