It looks like DevPi may just work as a caching proxy as well. I also found Bandersnatch too. https://github.com/pypa/bandersnatch which is a configurable mirror with allow / block lists.
Essentially want something like DevPi but add the package to the bandersnatch allow list and mirror it from then on. With some extra large packages in the deny list.
Probably possible to wire that all up reasonably well, but probably just using a caching proxy is 90%+ of the improvement anyways. So may just stick with that.
https://devpi.net/docs/devpi/devpi/stable/%2Bd/index.html https://blog.jcharistech.com/2022/08/13/how-to-run-a-pypi-mi...
another option is https://pypi.org/project/python-pypi-mirror/