> We build our own custom build system, because there is no CI that can do actual DAGs (maybe a few).

Have you had a look at GitLab CI? They have a bit of documentation here: https://docs.gitlab.com/ee/ci/directed_acyclic_graph/

Now, I don't work on any projects that are too complicated, but I recall that piece of functionality working as one would expect: https://docs.gitlab.com/ee/ci/yaml/index.html#needs

Also there's Drone CI, which also supports setting up dependencies in your pipelines, if you'd prefer something that's not connected to GitLab CI so closely: https://docs.drone.io/pipeline/docker/syntax/parallelism/