I am working on Grog, the “grug-brained” alternative to Bazel. A mono-repo build tool where all you do is provide your build commands and interdependencies and the tool will run everything in parallel while caching as much as possible.
Grog on the other hand let's you keep your existing build setup while just parallelizing and caching it. It's not a full replacement, but it's more than enough for most mid-sized teams that want to have fast mono-repo builds.