Take a look at job pools which are implemented in ninja.
You should be able to setup 2 pools and put the expensive target in one
pool and the rest in the other, which should give you the desired behavior.
https://cmake.org/cmake/help/latest/prop_gbl/JOB_POOLS.html
My current project builds a bunch (30) different plugins that do NOT depend on
each other. One of those plugins has about 200 files of highly templated C++
code to build which takes a while to compile and link. Meanwhile the other
plugins only have about 20-40 files of "run-of-the-mill" c++
