Robin doesn't have to be that young of a kid beyond the story Bruce first meets him. That's usually just used as kind of a lazy strawman/excuse to avoid bringing him in the movies. Jump ahead and do it like BTAS did and have him be 18-21 to Bruce's early 30s and it shouldn't have to be a tonal shift.
It wouldn't automatically make him resonate with audiences more, but it wouldn't automatically ruin anything either. We already have solo Batman in films where the villains resonate more with the audiences than Bruce, so there's no real solid proof that Bruce always works better without Dick or that their dynamic is unnecessary.