Recently I have quite often used the focus bracketing feature of my A7RV, and I am learning the pitfalls of it.
One problem I have is: you have to set via AF the nearest point which you want to be in focus and then start the bracketing. But sometimes this nearest point is at the very outer edge of the frame - especially when shooting macro or tele scenarios where DOF is rather thin. And that leads to a problem - the focus rectangle of the A7RV can't be positioned to the very edge!
You’re right that is a problem that I have found as well. My workaround is to go to manual focus and even use focus speaking to select an area in the corner and even front focus on that a little bit just to be sure. If you forget to switch back to AF focus it doesn’t matter as the focus bracketing will start working properly even if sent to manual focus. Too bad it does not reset to your initial focus point at the conclusion of the series as the OM system cameras do.
Since I have an A7R2, which has no focus bracketing feature - and also won't set the focus rectangle on the edge of the frame - I do it all manually, using focus peaking throughout the process. It works fine.
For focus stacking landscape type images, I leave it in AF, back-button focus is enabled. I move the green focus box to the bottom edge of the screen, on the near foreground, then I carefully recompose slightly to temporarily focus on the closest element in the foreground (or even a little closer), then revert the composition back to the original position and take the shot, without re-focusing.
