How do disable iOS keyboard from displaying?
Disabling the text insertion gesture may be impossible unless you consider adding an external keyboard or setting up voiceover to intercept and remap that gesture. I have had good success with peers using physical keyboards or accessories and propping up their iPad. Or use an Apple Pencil for many gestures and scribbles, which lessens the time fingers are on screen.
Hopefully over time you can train yourself and just summon the keyboard when you want it, knowing the gesture that summons it is a tap and hold.
Here are the global touch delays, if you want to experiment with them.

I find these are invaluable for teaching people to experience the sub second delays needed for a gesture to be recognized, but in the end, the penalty of slowing down the keyboard also slows down other interactions and most people don’t use these unless they need it due to dexterity conditions that make the default timing problematic and over sensitive / too reactive.