> At work we also cache all 3rd party dependencies locally [...]

Can you say a bit about your platform and tooling? Are you working in a single language or a polyglot world? Is the cacheing at the network level or are your build tools aware of your mirrors?

I work in a space (biotech/pharma/...) that shares these concerns. I've solved the Perl specific version with Pinto (https://metacpan.org/pod/Pinto) and the more generic version with Spack (https://spack.io) [which is neat also because it supports installing multiple versions of applications, doesn't require root, <other things>].