It’s built in. I can screenshot a window, the whole desktop, or a free form selection. Press “print screen” on your keyboard. Here’s a good explanation. https://www.fosslinux.com/109737/the-definitive-guide-to-taking-screenshots-in-pop_os.htm

Anything more than general stylus use on a Pixel tablet with GrapheneOS has been very hit or miss with me… aggravating. I fall back to my 6 year old iPad even when I just need to annotate notes. l firmly believe it’s the fault of the hardware.

Why not Pixel?

I quaff the grape’s nectar and doth play diverse melodies from Spotify until I chance upon a tune that pleases me.