SwiftUI MacOS Form Custom Layout

Question

I had an earlier question that I got awesome help to but there's something not quite right with the layout still. Figured I'd create a new question rather than continue that one.

I'm making a custom picker using a button and want it laid out like the other pickers, textfields, etc on my form. In the previous question I learned to use the alignmentGuide. However that isn't working as the field isn't quite lined up with the others AND I can only make the window a bit smaller and then it locks into place. I want it to line up with above and be dynamic to window size adjustments when running.

Here's what it looks like right now

This is as small as I can make it:

And here's the current code:

import SwiftUI

struct ContentView: View {

@State var myName:String = "Kyra"
@State var selectedPickerItem: String?
var pickerItems = ["item 1",
                   "item 2",
                   "item 3",
                   "item 4",
                   "item 5",
                   "item 6"]
@State var showingPopover:Bool = false
@State var selectedItems = [String]()
@State var allItems:[String] = ["more items",
                     "another item",
                     "and more",
                     "still more",
                     "yet still more",
                     "and the final item"]
@State private var commonSize = CGSize()
@State private var commonTextSize = CGSize()

var body: some View {
    Form {
        
        TextField("My Name:", text: $myName, prompt: Text("What's your name?"))
            .foregroundColor(.white)
            .background(Color(red: 0.4192, green: 0.2358, blue: 0.3450))
        
        Picker(selection: $selectedPickerItem, label: Text("Pick Something:")) {
            Text("No Chosen Item").tag(nil as String?)
            ForEach(pickerItems, id: \.self) { item in
                Text(item).tag(item as String?)
            }
        }
        .foregroundColor(.white)
        .background(Color(red: 0.2645, green: 0.3347, blue: 0.4008))
        
        
        HStack() {
            Text("Select Items:")
                .foregroundColor(.white)
                .readSize { textSize in
                    commonTextSize = textSize
                }
            Button(action: {
                showingPopover.toggle()
            }) {
                HStack {
                    Spacer()
                    Image(systemName: "\($selectedItems.count).circle")
                        .foregroundColor(.secondary)
                        .font(.title2)
                    Image(systemName: "chevron.right")
                        .foregroundColor(.secondary)
                        .font(.caption)
                }
            }
            .readSize { textSize in
                commonSize = textSize
            }
            .popover(isPresented: $showingPopover) {
                EmptyView()
            }
        }
        .alignmentGuide(.leading, computeValue: { d in (d.width - commonSize.width) })
        .background(Color(red: 0.4192, green: 0.2358, blue: 0.3450))
    }
    .padding()
}
}

// FROM https://stackoverflow.com/questions/57577462/get-width-of-a-view-using-in-swiftui
extension View {
  func readSize(onChange: @escaping (CGSize) -> Void) -> some View {
background(
  GeometryReader { geometryProxy in
    Color.clear
      .preference(key: SizePreferenceKey.self, value: geometryProxy.size)
  }
)
.onPreferenceChange(SizePreferenceKey.self, perform: onChange)
  }
}

private struct SizePreferenceKey: PreferenceKey {
  static var defaultValue: CGSize = .zero
  static func reduce(value: inout CGSize, nextValue: () -> CGSize) {}
}