Also ex-Google engineer and founded a bunch of startups in my time away.

I tried Bazel when it was first released as open-source but migrated away from it pretty quickly. It was just too complex for what it brought to the table. In a startup you should only be doing things that directly validate your business hypothesis, and futzing around with your build system is not really one of them.

I found it much more productive to use the native package managers and build systems that go with your language, eg. npm for Node, pip for Python, Gradle for Java or Kotlin.