PSA: On iPad to select text when in trackpad mode, double tap and hold as you slide.
And also for iOS 12 non 3D Touch devices (for now) you hold space bar and when in trackpad mode you tap the keyboard (with another finger) and it starts selecting text as you drag. (Tested on iPhone 6)
The iPad one might not be news to everyone but if one more person finds out that’s a plus.
Edit: iPad was tested on 11.4.1 so may change when iOS 12 releases