By X factor I presume you mean a combination of mental resilience and batting technique. Other than Root, and to a lesser extent Stokes, we don't have players with both so it's a question of which one you prioritise. As a manager I would always appoint for attitude and train for skill: time after time, I've seen it proven to be the better approach.
With regards why our coaches haven't sorted out Sibley and Burns's batting, maybe you're crediting them with too much initiative? Moeen Ali doesn't think too highly of them:
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India worked out that if you put the ball just above waist height on off stump 9 times out of 10 he will pull it through midwicket in the air. I can't believe our elite coaches haven't been able to work with him to rectify this.
Test cricket is an arms race between the opposition analysts and bowling coach and your batting coach. There's no batsman in the world who will walk into a Test team as the finished article. None of our batsmen ever seem to improve while they're on central contracts.