If I am allowed to make another suggestion (always hesitant to make them, in case it’s already possible and I’m to stupid to realise it): an option to select a system break in Engrave mode, and then move that break to the right or left by one bar (and - in analogy - to move frame breaks up/down by one system, I suppose). Would make lay-out tweaking a lot faster, I reckon…
Your wish is my (partial) command. In write mode, select the break signpost, and press Alt-L/R arrow.
Nice! Though indeed partial, it would be nice to have this in Engrave mode. Just trying this out, and it reminds me that it would also be nice to have a user option to force all system breaks to bar lines.