I made a really simple helper for using Dynamic Type with your own fonts

FontSizesHelper.swift

struct FontSizesHelper {
    static func dynamicSize(for originalSize: CGFloat, category: UIContentSizeCategory = UIApplication.shared.preferredContentSizeCategory) -> CGFloat {
        let modifyBy: CGFloat = {
            switch category {
                case .extraSmall: return -3
                case .small: return -2
                case .medium: return -1
                case .large: return 0
                case .extraLarge: return 2
                case .extraExtraLarge: return 4
                case .extraExtraExtraLarge: return 6
                default: return 10
            }
        }()

        let minPossibleFontSize: CGFloat = 8
        return max(minPossibleFontSize, originalSize + modifyBy)
    }

    static let sizes: [UIFontTextStyle: [UIContentSizeCategory: CGFloat]] = [
        .headline: [
            .extraSmall: 17,
            .small: 18,
            .medium: 19,
            .large: 20,
            .extraLarge: 21,
            .extraExtraLarge: 22,
            .extraExtraExtraLarge: 23,
        ],
        .subheadline: [
            .extraSmall: 15,
            .small: 16,
            .medium: 17,
            .large: 18,
            .extraLarge: 19,
            .extraExtraLarge: 20,
            .extraExtraExtraLarge: 21,
        ],
        .body: [
            .extraSmall: 12,
            .small: 13,
            .medium: 14,
            .large: 15,
            .extraLarge: 16,
            .extraExtraLarge: 17,
            .extraExtraExtraLarge: 18,
        ],
        .caption1: [
            .extraSmall: 12,
            .small: 12,
            .medium: 13,
            .large: 14,
            .extraLarge: 15,
            .extraExtraLarge: 16,
            .extraExtraExtraLarge: 16,
        ],
        .caption2: [
            .extraSmall: 10,
            .small: 11,
            .medium: 12,
            .large: 12,
            .extraLarge: 12,
            .extraExtraLarge: 13,
            .extraExtraExtraLarge: 14,
        ],
        .footnote: [
            .extraSmall: 10,
            .small: 10,
            .medium: 11,
            .large: 11,
            .extraLarge: 12,
            .extraExtraLarge: 12,
            .extraExtraExtraLarge: 13,
        ]
    ]

    static func size(for style: UIFontTextStyle, with contentSizeCategory: UIContentSizeCategory) -> CGFloat {
        return sizes[style]?[contentSizeCategory] ?? 0
    }
}