I understand that some other OSes may have more convenient methods of taking scrolling screenshots.
The functionality in GrapheneOS is provided upstream from AOSP, and I doubt the small development team in GrapheneOS would wish to change it.
You could always contact them on GitHub to ask, but they tend to focus on privacy and security features, and where they do work on accessibility features, that is mostly for things which are broken in AOSP. However, the scrolling screenshot feature from AOSP works fine.
I personally am able to screenshot with one hand: Thumb on left of the phone, index finger on power button, and middle finger on volume down button, although most of the time I am two-handed.
For standard screenshots I use the recent screen which doesn't require any pressing of the power/volume buttons, and is thus easier one-handed.