I would like the hole to be as small as possible. I have a solid steel rod on hand (ø=12mm) but I could even get a smaller one. This would be the least intrusive on a structural wall (brick), which technically I’m not supposed to touch without planning permission. I would bend that rule for a rod ≤12mm. I’m not sure if removing a whole brick is bit risky since it’s an old house and the state of the brick is probably dodgy to begin with.