Probably not the highest prio, but this would be nice and consistent with the standard UI behavior.

Try cntl + T. its only there for developmental purposes.

I had it close with ESC, but people said they wanted to keep it open on the side, so I made it not close with esc

I know there is already a hotkey for the GUI and this one is a bit nick-picking, but to me this behavior seems not factorio native and inconsistent with other UI elements.

Just my 2 cents: Esc should close the GUI and keeping it 'pinned' would fit better as an option, i.e. following the principle of least surprise :)