Selecting rather than highlighting in iPad books
On iOS 14 there is no preference you can set in the settings to disable highlight function in general or to change the gesture itself.

If your device gets bumped often or your touch this triggers highlights too fast, you can enable accessibility to increase the hold detection time and also keep swipe gestures fast to keep page swipes more responsive but the highlight touch time delayed. This is a system wide setting, so you may not like it elsewhere.
I recommend reading about all the gestures that change if you enable the top most gesture. It changes more than just the two toggles I show in non-default settings above in the screen image.
https://support.apple.com/en-us/HT205269