It is still not obvious to me what you are exactly looking for.
The Divide tool, as its name says, is to divide a single entity into two. I use this usually on intersections or long lines which have to be trimmed in the middle.
From your screenshot it looks like you have a bunch of blocks. For this job use the Explode tool, which is used to separate entity containers like polylines or blocks into their primitive entities. The primitive entities are not modified, but they are not glued together anymore.
So if the selected entities are blocks, you can use the Modify -> Explode tool (see
https://librecad.readthedocs.io/en/latest/ref/tools.html#modify at the end of the table) to separate them all at once.
But if you want all selected entities divided at there intersections, then there is indeed no tool in LibreCAD and you have to do it one by one.
Axelrodde wrote
and unite back together en mass
This is also not clear to me from your example. Is the whole selection one block or are there several blocks or polylines? So is it one big block to unite or do we talk about a bunch of smaller groups to combine?
Maybe, when you tell us the purpose of your drawing, someone may have a better idea how to achieve it with less effort.
investing less than half an hour into Search function can save hours or days of waiting for a solution