And make -j will execute the rules & build targets in parallel. Once I built a general batch parallel system using make that could pause & resume parallel jobs by using each job's log file as the make target. Then I discovered gnu parallel and scrapped my project.
Ha! Crazy, I didn't know that, thanks for the pointer. I recall having some issues with scaling to very large jobs, when I got lists of targets too long for make to deal with. (I think... I could be mis-remembering the details, but something in my pipeline would fail with really big batches.) Maybe parallel split away from make when it ran into similar issues, I wonder.
Dependencies in batch resource managers for distributed parallel jobs (or simpler ones) are standard practice. https://arc.liv.ac.uk/SGE/htmlman/htmlman1/qmake.html is an old adaptation of GNU Make which works within a job.
Gah story of my life - hours/days of grueling work trying to solve a problem, then someone say 'why don't you just use $tool_that_does_exactly_what_you_want_but_better?'. Code goes in the bin.
