A reference for what to do if WinUIEx isn't enough. Just install CsWin32, add the dpi method to NativeMethods.txt

Win32WindowHelper.cs

using Microsoft.UI.Xaml;
using System;
using System.Runtime.InteropServices;

internal partial class Win32WindowHelper
{
    private static WinProc? newWndProc = null;
    private static nint oldWndProc = nint.Zero;

    private POINT? minWindowSize = null;
    private POINT? maxWindowSize = null;

    private readonly Window window;

    public Win32WindowHelper(Window window)
    {
        this.window = window;
    }

    public void SetWindowMinMaxSize(POINT? minWindowSize = null, POINT? maxWindowSize = null)
    {
        this.minWindowSize = minWindowSize;
        this.maxWindowSize = maxWindowSize;

        var hwnd = GetWindowHandleForCurrentWindow(window);

        newWndProc = new WinProc(WndProc);
        oldWndProc = SetWindowLongPtr(hwnd, WindowLongIndexFlags.GWL_WNDPROC, newWndProc);
    }

    private static nint GetWindowHandleForCurrentWindow(object target) =>
        WinRT.Interop.WindowNative.GetWindowHandle(target);

    private nint WndProc(nint hWnd, WindowMessage Msg, nint wParam, nint lParam)
    {
        switch (Msg)
        {
            case WindowMessage.WM_GETMINMAXINFO:
                var dpi = Windows.Win32.PInvoke.GetDpiForWindow(new Windows.Win32.Foundation.HWND(hWnd));
                var scalingFactor = (float)dpi / 96;

                var minMaxInfo = Marshal.PtrToStructure<MINMAXINFO>(lParam);
                if (minWindowSize != null)
                {
                    minMaxInfo.ptMinTrackSize.x = (int)(minWindowSize.Value.x * scalingFactor);
                    minMaxInfo.ptMinTrackSize.y = (int)(minWindowSize.Value.y * scalingFactor);
                }
                if (maxWindowSize != null)
                {
                    minMaxInfo.ptMaxTrackSize.x = (int)(maxWindowSize.Value.x * scalingFactor);
                    minMaxInfo.ptMaxTrackSize.y = (int)(maxWindowSize.Value.y * scalingFactor);
                }

                Marshal.StructureToPtr(minMaxInfo, lParam, true);
                break;

        }
        return CallWindowProc(oldWndProc, hWnd, Msg, wParam, lParam);
    }

    private nint SetWindowLongPtr(nint hWnd, WindowLongIndexFlags nIndex, WinProc newProc)
    {
        nint funcPtr = Marshal.GetFunctionPointerForDelegate(newProc);
        if (nint.Size == 8)
            return SetWindowLongPtr64(hWnd, nIndex, funcPtr);
        else
            return new nint(SetWindowLong32(hWnd, nIndex, funcPtr));
    }

    internal struct POINT
    {
        public int x;
        public int y;
    }

    [StructLayout(LayoutKind.Sequential)]
    private struct MINMAXINFO
    {
        public POINT ptReserved;
        public POINT ptMaxSize;
        public POINT ptMaxPosition;
        public POINT ptMinTrackSize;
        public POINT ptMaxTrackSize;
    }

    internal enum WindowMessage : int
    {
        WM_GETMINMAXINFO = 0x0024,
    }

    [Flags]
    internal enum WindowLongIndexFlags : int
    {
        GWL_WNDPROC = -4,
    }

    [DllImport("user32.dll")]
    internal static extern IntPtr CallWindowProc(IntPtr lpPrevWndFunc, IntPtr hWnd, WindowMessage Msg, IntPtr wParam, IntPtr lParam);

    [DllImport("user32.dll", EntryPoint = "SetWindowLongPtr")]
    internal static extern IntPtr SetWindowLongPtr64(IntPtr hWnd, WindowLongIndexFlags nIndex, IntPtr newProc);

    [DllImport("user32.dll", EntryPoint = "SetWindowLong")]
    internal static extern int SetWindowLong32(IntPtr hWnd, WindowLongIndexFlags nIndex, IntPtr newProc);

    internal delegate IntPtr WinProc(IntPtr hWnd, WindowMessage Msg, IntPtr wParam, IntPtr lParam);
}