So introduce him as 12-14 and skip ahead to 18. Bruce would be in his late 20s/early 30s (he generally should only be around a decade older than Dick) and people in that age range don't visibly change that much in general, especially in a Hollywood production.
If the audiences weren't bothered seeing Michael Cain not age a day in Batman Begins when we saw Bruce go from a child to Christian Bale, I don't see anything breaking suspension of disbelief here. We also don't even need to touch on an origin at all aside from some flashbacks or exposition. Just do what BTAS did and drop us into a movie with them both at the ages they need to be.
Keep in mind the "great movies" he's been getting usually cite Bruce as the weaker/blander part compared to the villains. A common criticism of the 1989 film at the time was that it should have been called "Joker" instead of "Batman." Alfred and Gordon, for supposedly being more integral, aren't really big selling points on those either. Yeah, WB doesn't have incentive to break from their formula, but that's not always indicative of a good thing. They don't have much incentive to break from Evil Superman either, do they?
Though Superman is an easier sell than Robin, and they keep fucking that one up. If Robin is to be left out of the movies, it's not because he's not a good character or integral, it's just probably for the best WB doesn't cast their gaze on him because God knows what they'd do.