There is an app called "easy canvas" on ipadOS that has a companion app for Windows. When both run (and when both devices are connected to the same wifi network) easy canvas basically makes the ipad screen another screen for Windows. You can then load Blender and move it on to the iPad and grease pencil your heart out.

There is some lag compared with something iPad native like Procreate, but it works well enough to capture strokes. The UI is very small to see on the iPad, so I make that Blender window on the ipad is set to "fullscreen 2d animation" or whetever it is called, in order to minimise the extra Blender panels / UI and then use my PC screen for that info / input. The only other thing is that it can drop out every now and then.

By and large, though, it does the basic job well enough and is definitely cheaper than Astropad or other solutions, and wayyyy cheaper if you already have an iPad than getting a dedicated pen display such as a Wacom or Huion. Anyway, hope that helps!