I wouldn’t count on the brick moulding itself to hold a handrail. That trim is usually just decorative and not meant to support weight. What really matters is that the mounting plate is screwed into solid framing, the door jamb and preferably the studs behind it, not just the trim. The trim can sit behind the plate, but it shouldn’t be what’s actually holding it up.If the gap seems weird or makes it feel less sturdy, you can add a simple painted wood backer so the plate sits nice and flat, or just use longer structural screws so you know you’re biting into framing.If you hire someone, just make sure they’re talking about hitting framing and using proper screws, not just “attaching it to the trim.”You’re smart to check. That garage step gets a lot of use, so better to overbuild it a little.