The build will always run out of ram or swap thrash if you start it with too many jobs.
We can use a jobserver to fix this, here is one: steve - Gentoo wiki
I wanted to port it but have not gotten around to it, the jobserver will dynamically reduce the number of available compile jobs as ram becomes more scarce and so. 
It basically needs a) port to userlandfs OR fuse2 (they use fuse3) And an implementation for how we check for RAM, their implementation is based on /proc which we do not have.
EDIT: Now Iām wondering if this is a job we could make the roster do