Example of how to get Dynamic Type with custom fonts in SwiftUI

SwiftUI+CustomFonts+DynamicType.swift

import SwiftUI

/// Example of how to get Dynamic Type with custom fonts in SwiftUI.
struct ContentView: View {
    var body: some View {
        VStack(spacing: 20) {
            Text("A large title").customFont(.largeTitle) // "Optima-ExtraBlack", 28
            Text("A body").customFont(.body)              // "Kailasa", 16
            Text("A caption").customFont(.caption2)       // "IowanOldStyle-Italic", 11
        }
    }
}

extension View {
    func customFont(_ textStyle: UIFont.TextStyle) -> Self.Modified<CustomFont> {
        return modifier(CustomFont(textStyle: textStyle))
    }
}

struct CustomFont: ViewModifier {
    let textStyle: UIFont.TextStyle

    /// Will trigger the refresh of the view when the ContentSizeCategory changes.
    @Environment(\.sizeCategory) var sizeCategory: ContentSizeCategory

    func body(content: Content) -> some View {
        guard let fontDescription = fontDescriptions[textStyle] else {
            fatalError()
        }

        let fontMetrics = UIFontMetrics(forTextStyle: textStyle)
        let fontSize = fontMetrics.scaledValue(for: fontDescription.1)

        return content.font(.custom(fontDescription.0, size: fontSize))
    }
}

/// Define the custom fonts to use, depending on the TextStyle.
typealias CustomFontDescription = (String, CGFloat)
private var fontDescriptions: [UIFont.TextStyle: CustomFontDescription] = [
    .largeTitle: ("Optima-ExtraBlack", 28),
    .body: ("Kailasa", 16),
    .caption2: ("IowanOldStyle-Italic", 11)
    // ... other styles
]

My take is that UIFontMetrics scaledValue responds to current dynamic type setting, but not the SwiftUI sizeCategory environment.
So this ought to work in a real app although it would be even better with a solution that worked with 'pure' SwiftUI.