One thing your tool appears to be missing (IMO) is execution sandboxing. This is useful, as you likely know, for avoiding undeclared dependencies and for avoiding dirty builds due to actions polluting the source directory, among other things. I was playing around with allowing configurable sandboxing, with symlink forest and docker as two intial options.
Very cool that you are also recognizing this issue and working on it. I sent you an email in case you want to exchange further.
[0] https://www.pantsbuild.org/stable/docs/introduction/welcome-...
Come back when you have millions of lines of code, written over decades by hundreds (or thousands) of full time developers.
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.
https://grog.build/why-grog/